As far as I checked through private server database there are defined stages in main composing which state what quality minor you need and what are the results of composing.
If you have the database, have a look at cq_eudemon_rbn_rqr table. It also defines if there will be a roll or not, failure rate, loyalty needed, if the race requirement comes active at this stage etc.
So, to my knowledge so far the main composing works like this: the max defined * at the pet from main growth is divided evenly over defined stages (you can calculate the max * possible from cq_pet_grade). For every stage there is a defined minimum and maximum you can get. I don't know how the result is picked, but it could be that minimum is if the minor barely fits the requirements (like, you need 2* minor, but actually minor has no main attribute growth at all and those 2* on minor you got with orbs), maximum is if minor fits requirements because of main attribute growth only.
I am shooting blind here, that is just my guess.
After server has picked what * you can get on this composing it will divide those starmarks over individual growth rates and make a roll (if there is any - at the very low stages there is no roll and you can also disable the roll with clover) on those individual growth rates.
Say, you need level 50 1* minor and defined results are 27-41 starmarks.
If it is BloodSage (which can fit that 1* requirement because of rarity only) with 0 marks (not sure tho regarding this part, just a guess) from main attribute growth, server will pick 27 for result, then split it between main attribute growth evenly and pass it roll.
But let's say you are composing MageAtkDef which has no rarity and composed 1* minor from scratch and it has 100 marks from main attribute growth. This time server will pick 41 marks for result before dicing.
So there are 2 things to remember:
- making 10* minor instead of 1* is a waste
- once it starts asking for 6* minors it is not worth making a better minor than needed because the difference between min and max result does not compensate what you have to invest to make "best fit" minor. Means, you will get better results if you make 2 minors which barely fit requirements instead of 1 which will give you max result. In both cases you will invest about the same, but say, with 2 bad minors you will get 2x18 marks (+2 reborns), with 1 good you will get 27 +1 reborn.
Level of minor affects nothing, only the growth rates.