Hey Andy, how much braid you put on your reel is totally up to you. I do it two ways…
When the line is brand new, I tie the line directly to the spool, then put a 1/2 inch square of electrical tape over the knot, taping the line to the spool. This will prevent the line from slipping on the spool under pressure. Then crank on the entire spool of braid and go fishing.
Then, once the line has been used for a season or two, and I've cut/retied often enough to reduce the line on the spool, then I do something different.
I'll get some mono line and using a Uni – Uni knot, I'll connect the two lines together. Then I'll crank on enough mono to fill the spool back up. Now here comes the tricky part.
I have several different reels of the same brand/size. I'll tie the mono to the spool of the new reel and crank it from one reel to the other, effectively reversing the line. Now I've got brand new PowerPro on the fish end of the line and a full reel spool.
So while PowerPro or High Seas braid isn't cheap, because I've gotten two or more years from one filler spool of line, when I now need to throw it away, I'm fine with that because it's now cost me about $7.00 per year! So it's actually much cheaper to use braid than quality mono line!