When refining them, just do 100 at a time. Back out of the clothier table and go back in like a fresh start, for some reason, when i stand and do thousands at a time., the percentages of green,blue,purple and yellow upgrade mates, drop drastically, im more or less guarenteed one or more yellow from doing 100 at a time.
I dont have any facts to back this up, just that ive done thousands upon thousands and this is how i maximised my drop rate. Have a go, let me know how you get on.