For backing I use plumbers Teflon tape. It works well and doesn't stick to the spool. A couple turns around the spool is plenty, then I fill the reel with the lb braid I want to use.
Just wrap the tape around the entire spool then wrap the line around 3-5 times to make sure it doesn't slip and pull the line very tight so it digs into the tape to some degree.
I like between 200 and 300 yards of braid on all of my saltwater reels. I'm not a fan of these 100-150 yard spools, they cause a lot of trouble. If it's a big reel and I can do 300 yards and still have plenty of room only then I'll put mono under it and connect both lines with a small
Yucatan knot or a simple to tie
Uni-to-Uni Knot. You don't have to do that if the reel barely holds 200 yards, then obviously just give it a wrap of tape.