If you need to maximize the amount of backing on the reel, you can remove the fly line and then cut some of the running line off the back end of the fly line. Unless you routinely carry more than about 70-80 ft of line in the air while casting, you shouln't notice a difference with your casts. Removing 20 or so feet of fly line will allow quite a few yards of backing to stay on the reel.

regards, Keith