how much backing and what size

[fishfair]

on my reel it say to fill use 6lb mono 200 yardsI am getting ready to spool my spinning reel with 15lb /4lb dia. power pro braid ,is their any set rule on how much backing or line size to use .

[Timbo]

Some people put about a 1/4 spool more or less of mono on their spool first before putting on the braid. Part of the reason is because braid can "spin" on the spool since it is so slick. I try to use mono close to the size of the braid.
For instance, 10 lb mono with what size braid you have. It doesn't really matter that much as long as the knot you use to connect the lines is strong. Those who want to fill the entire spool with braid often put a couple wraps of electrical tape on the spool so the braid has something the grab on to and so it doesn't slip. Thats what I usually do. Another reason to put mono on first is to save money on the braid. After a couple three years I reuse the braid by putting it on another spool! No set rule that I know of, its what works for you! Hope this helps.
BTW, you didn't say what size reel.

[Fishman]

Try using 30lb. Dacron backing. As to how much backing to use is a guessing game because of line diameter and pound test? For the most part I like to use fifty yards of backing for most of my reel depending on the size of the spool.