My understanding on maximizing casting distance with baitcasters and lighter lures is keeping the spool lighter. Hence spools that are themselves lighter and or shallower. This leads me to wonder when applying backing to a normal spool, is it better to use heavier mono (20+) and take up more space per rotation or lighter 12-14lb. I like to use 20-30lb braid to fluro leader so i dont like overpacking the reel with braid. A. Its a waist of line. B. I want to minimize the amount of digging in the braid will do under tension. I usually dont put on much over 75 or so yards of braid. I may be way off base but i was thinking about it tonight.

Weight wise it doesn’t matter. I like 10# backing. It doesn’t take a lot, packs well and make a small easily buried knot. 

No question, 20-25-lb mono for backing - all you'll need for backing is 20-25 yds.