Grinder is a calculator for determining potential outcomes based on skill, difficulty, bonus and tool quality (if applicable).

Manytox Explains (extracted from Discord and edited):

Regarding casting power: for this you have to understand the bowels of most wurm's rng-iness. Skillchecks! I invite you to play around with this tool: https://forum.wurmonline.com/index.php?/topic/163777-wurm-skillcheckskill-gain-simulator-wow-neato/ (warning. Don't use them as reference directly).

Unless you know what you are doing, you don't know all the input variables. Namely, difficulty and bonus. Check mining tab for example and see it does a lot of stuff.

In any case, a skillcheck is a complex mathematical formula wurm uses to produce a probability function with a specific distribution based on a mixture of these various inputs. As a base, it checks for the value of the skill in question (hence skillcheck, duh), a 'difficulty' value that's set per action (for spells it correspond to the same casting difficulty you can see in the spell list) and 'bonus'. Skill and Diff are the main metric of the formula, while bonus shifts the distribution a little.

It's more like “here is a normal distribution, now make it go crazy”. Also by default the output of the skillcheck is in the range of [-100,100].

If you input random numbers and see what happens you can see the values are only between -100 and 100. Depending on what the skillcheck is meant to do it later gets interpreted for other shit. For example, in crafting, negative results means you failed to craft. In channeling, below 0 you start seeing the messages that you failed blah blah blah, if its low enough it starts damaging the item and at a threshold that depends on QL of the item and that gets modified by runes and item material it just destroys (shatters) it. The values over 0 are just the cast power.

A probability distribution has a lot of properties, but you can you gather by playing around with the tool without knowing much that it is quite simple and straightfoward: the more skill and bonus you input, the more the curve shifts to the right (meaning higher chances of succeding to craft/output QL or succeeding to cast/cast power). Likewise, the higher the difficulty, the more it shifts to the left. Yes, difficulty makes things be harder to cast. With enough playing around you can gather that difficulty is quite punishing. The tool lets you drag a range to see the combined probability of it, try checking from -100 to 0 (failing chance) as you increase difficulty. With 90 skill and 70 bonus in channeling, a 90 diff spell is twice as likely to fail as a 80 diff one. Same for 80 diff compared to 70 diff, which explains why the harder spells are so shitty to cast and unforgiving.

So, now that we know the variables, we can try to min-max everything to maxizime our chances of casting, and most importantly, getting high casts. Skill is obvious, difficulty is non-modifiable for spells except…

There are two enchants that reduce difficulty. Well, three really. You have efficiency, a tool enchant that reduces difficulty by 0.05 per 1 cast power. 100 eff reduces diff by 5, which is rather dramatic if you ask me. Titanforged does the same and adds to it, but by half that rate.

However those are tool enchants, great for skillchecks that involve tools as a input variable (which we ain't touching with a three meter pole for now). Channeling doesn't use the statuette as a tool, hence why in vanilla nothing of the statuette matters at ALL (material, rarity, ql, color, maker, you name it).

Hence the only way we can minmax this variable for channeling is via the industry enchant, for jewelry. It works at 1/10 the rate of efficiency, so at 100 cast power its 0.5 less diff. Not irrelevant, but not groundbreaking (they don't stack with multiple jewelry, so just have 1).

The spellcraft mod, the same one that introduced these three enchants has a feature to make 1) QL and 2) rarity of the statuette influence the input variables of the skillcheck.

QL adds to bonus. Bonus is always capped at 70 no matter what, so between QL and altar bonus its extremely easy to get it high. It's why I said alignment doesn't help much, as it adds a bit to bonus, but not that much, and bonus already doesn't shift the curve a lot.

Standing near an altar gives you bonus bonus. You will be usually doing enchanting runs next to one because you have to sac.

Rarity is where the cake is at. It increases the final result of the skillcheck (not the input variables (!!!)) by a flat rate, it being rarity x 3 (3 for rare, 6 for supreme and 9 for fantastic).
You can see the benediction 'flat +5' thingy in the grinder tool. It works the same way, if you care to test. So, with a fantastic statue, your max cast goes from 100 to 109.

Now, if you are smart and know a bit of wurm online channeling you'll scream at me 'YOU MUST BE WRONG! Before benediction bonus we still could create enchants up to 104 power! Or 'The other day Giai got 110 expand on a bsb, how come if 109 is the limit?' This is the funniest part, because this goes contrary to an extremely popular belief WO and even us had back then. It was said that you have better chances of having a higher enchant if you dispell it and then cast it again. Totally wrong. There is an enchant upgrading bonus. For each 20 points you improve an already existing enchant, you get an extra 1 cast power. Hence why you got a lot of 104s (105s were highly improbable).

A couple of years ago they changed it in wurm to make the old rumor true, making an unenchanted item count as having '0' cast power, so it always got the best improving bonus, but it still applies to us. With that it makes the maximum cast you can get here 113, theorically borderline 114 (never) with a fant statue getting a 100 roll on an item with near 0 cast power in it.

Also, important note, cast power has decimals. When you examine an item, it shows an integer; that's just an approximation. For example, to have expand make a crate rack hold 100 crates you need 76.45 something expand, Hence, a few 76 work and some don't, but all 77 upwards do (76.5 and above).

That's pretty much all I can tell you without messing more deeper with the skillcheck wogic honestly. The bonus variable is easy to get to max, a 90ql statuette makes do very well for that

With that information you can more or less use the fixed bonus (channeling) simulation of the tool to estimate your odds. Input your skill, put the difficulty of the spell, set bonus to 70 assuming you have that maxed out. Like, if you have a fantastic statuette and you want to see what are your odds of getting a 90 cast from an unenchanted item, select 81 to 100 instead of 90 to 100. Adding the improve bonus requires meddling with the ks file, which is meh.