Here is a little chart/reference guide from Mr. Bob Clouser and the folks at Temple Fork Outfitters. No matter which rod you use, this should help you match up the rod/line size to the flies you will be throwing.
We must understand that a fly line is used to move weight forward to the end of the cast, whether it be a hook, small dry fly or a heavy weighted streamer or nymph.
Fly lines are measured in weight by grains and this grain weight is used to move weight forward. For example; a 5 weight fly line weighing 140 grains is not capable of pulling the same weight thru the cast as a 210 grain 8 weight does. There are many variables to consider while casting such as wide open loops, chuck and duck, over powering or lobbing.
This chart is designed upon the ease of the cast plus normal tight loops that will cut the wind with ease. An oval back cast along with the use of the body is a must when casting weighted flies.
We will start with a 5 weight fly line, anything under that is specially designed to cast small light flies and is not suited for any type of weighted fly.
Lead Eye Weights most suitable for these line weights.
Line Ounce of weight
5 weight = 1/120, 1/80, 1/50
6 weight = 1/120, 1/80, 1/50
7 weight = 1/120, 1/80, 1/50
8 weight = 1/120, 1,80, 1/50, 1/30
9 weight = 1/120, 1/80, 1/50, 1/30, 1/24
10 weight =1/120, 1/80, 1/50, 1/30, 1/24
The above listing of course is not written in stone but if the formula is followed, long easy casts can be made. In many instances a heavier eye weight than listed can be used but it will test the caster and the capability of the rod and line.
Note” The above suggestion for choosing the right weight a fly line can move forward with ease has it variations of course, most variations comes with the style of casting being used. The most proficient style developed by Lefty Kreh where the body is involved in the cast will make casting weight more efficient.