I waste FG on 50FG gacha if there is one or more 5* I want. There is a small chance of rainbow and anyway, I don't have enough FG for enough Rainbow Shards.
I'll do 30FG gacha only if there is a chance of a 5* I really want. Or for dupe of a 5* I really want.

Salt is salty anyway. 50FG or 30FG gacha is the same. One will never give you a rainbow 'cause it's a bitch, the other will never give you the 5* you want 'cause it's a bitch too.