EDIT: Solved, see my own reply below OP

Hey! I'm struggling to find what endurance actually does to HP growth on level-up in MM1. I want to figure it out so I can more accurately judge what's a good HP roll, and what I should reload the save to reroll.

I know each class has a base range for HP growth, for example 1-12 for the knight. So how does endurance affect that? Does it add a fixed bonus? Does it change the max cap, but not the min cap?
How much does it change? Endurance / 4? Endurance / 5?

I know this is very nitpicky, but I'm just too obsessive to keep playing and accepting HP rolls until I know if they're good or not lol.

I've done some testing by leveling and reloading multiple times, with both a knight and a sorceror, and using a hex editor to edit Endurance stat to try leveling at difference Endurance levels.

Knight has a base HP growth of 1-12, and Sorceror has a base growth of 1-6

With 18 endurance, both the knight and sorceror never grew their HP by less than 4. Additionally, the Knight never grew by more than 15, and the Sorcerer never grow by more than 9.

I then tried leveling the knight with 20 endurance, and managed to get HP to grow by 16, but never less than 5.

With that in mind I think I can conclude that the effect of Endurance on HP growth in MM1 is to add a fixed value equal to Endurance / 5 rounded down.

Since 18 / 5 is 3.6, rounded down that would be 3
20 / 5 is exactly 4

Which tracks with my findings that 18 gives a fixed bonus to HP growth of 3, and 20 gives a fixed bonus of 4.

It's technically also possible for the effect to be Endurance / 6 rounded up, but it just feels like a really odd choice and I see no reason why the developers would program it like that. They're obviously inspired by D&D, in which the convention is for modifiers to be the stat divided by some number and then rounded down. So I'm assuming this is the case here too.

The AD&D convention at the time was different: Each stat would use a table, with the table being different for each stat. Typically there's be a "dead" zone right in the middle of the range, so there might not be much (if any) mechanical difference between 7 and 14 in the stat.

Might & Magic games have always used a table of sorts. Typically it might look something like:
* 10-11: +0 (lower would yield a penalty)
* 12-13: +1
* 14-15: +2
* 16-17: +3
* 18-19: +4
* 20-24: +5 (notice the bigger range?)
* 25-29: +6
* 30-44: +7
* 45-59: +8
and so forth. This means that, once the attribute gets very large, you need more points to get the bonus. Of course, in MM1 you're not going to get your attributes that high, but it becomes feasible later in the series.

Your observations indicate that 18 is +3 and 20 is +4 in MM1. You could try other values, as well.

By the way, the same table is likely used for other attributes. Intelligence and Personality are easiest to check, because (at least in MM2 but I'm guessing MM1 is the same here) the effects are retroactive; for full casters (Sorcerer and Cleric) the game just takes a constant, adds a value based on race, adds the Int or Per bonus, and then multiplies by the character's level. So, you could check by hex editing the character's base Int, then sleeping, then checking the character's SP.

(There's a distinction between base and current; current is what you see on the status screen, but base is (I assume) used for HP growth and SP. When you rest, current is set to base.)

Aaah, that's a great tip! I did notice that SP immediately goes up based on INT after a rest, so if it is the same table that seems like a much quicker way of determining than my brute forced method of leveling up again and again.

I might give it a try some time and make an table for stats at 8-30! If I do, I'll come back here and update the thread :)