kinda necroing this but yea i just noticed this too
I poked around the code and managed to understand how it works (at least that part), adjusted the values so the last quality is just a tad better than vanilla, since honestly legendary vanilla big miner is just too good
you need to change the "mining_drill_resource_drain_multiplier" for each of the base qualities and new ones, what i did is reduce from .85 (mod's uncommon value) by .085 for each step and you end up with a godly of 4% drain
funny think i noticed but got lazy to check is 40Q means a drain of 0.000...1 (or something like that) making patches infinite (if there is not some kind of backup calculation since this would divide by 0 according to the formula i found