When using backing how much of the spool should be filled with backing? What is the best way of judging how to leave enough room so that you can put on 80-100 yards of the line you want to fish with but not over filling or under filling the spool?

what i do is i just spool on about 1/4 of the spool with backing and do the other 3/4 with my braid. if you really wanted to conserve braid (or whatever line your using that requires backing) you could spool up most of the spool with backing and then spool on 2 or 3 good casts worth of braid.

When using backing how much of the spool should be filled with backing? What is the best way of judging how to leave enough room so that you can put on 80-100 yards of the line you want to fish with but not over filling or under filling the spool?

I use spinning and I just judge it based on experience. But if you want to be precise, use a spare spool, put your 80-100 yards on first and fill it with backing to within 1/8" or however far you fill it. Then reverse things and transfer the line to the spool you'll actually be using.

I fill up the spool about 1/4 or 1/3 of the way depending on the size of the spool. After the backing is on I'll put some tape around it.