I used to install the fly line and backing on reels at the Sportsman's Warehouse. This is done for free and either 20 or 30 pound dacron backing is only 2 cents per yard. It is a rare reel that can take over 100 yards of backing. That is only $2.

For installation, you can either install your line and then the backing...then reverse everything, OR apply the backing to within 1/2" of the outer diameter of the normal width spool, then apply your fly line.

Most reels are quite accurate in estimating the amout of backing needed (figure they are suggesting 20 #). But some manufacturers like Okuma haven't got a clue. Some of their reels are 100 yards when they suggest 100 yards, and some only hold 40 yards when they suggest 100 yards. Go by the 1/2" rule.