You really do not want 10# PP on a baitcaster, it will dig in on itself and cause you some real headaches! Its a fantastic line for spinning reels though.
I prefer 30# for baitcasters, its more than sensitive enough for anything you want to do and casts a mile.
Take your rod to a park or a big backyard. Put a stick in the ground and tie the end of your line to it. Walk off the entire spool. Now put 50 yards of 10-12# cheap mono on your spool. Now tie on your braid to the mono using a uni-uni knot. And spool up 75 yards of braid and cut it. Look at the reel, is it to the level you want? If not, tie it and walk it off again and put more mono backing on. Keep doing this until the reel is where you like it. Once you do it once you should be able to visually see how much backing you will need for future reels. You should never need more than 75 yards of braid, its plenty for any casting you plan to do and will leave room for a years worth of breakoffs and reties.
I good way to get exactly 75 yards is to tie it off to the stick and walk it all off. Then grab the other end and tie it to the same stick. Now grab the line and walk until its tight. If you cut it there, it will be exactly half of your spool (75yds).