As a starting point, look at the grain window of the rod, add the low number and the high number and divide by 2 and you get the mid point of the grain window. That's a good place to start. After that, you start trying different lines with different grains to fine tune a line that also fits with your casting style. Some people like a heavier line to feel more load, others like less. As an example, a TFO Deer Creek 5/6 is rated 350-550 and the mid point is 450. I tried a 450 didn't like it one bit (although I've read guys who think it's great) and I've lined mine with a 360 compact skagit and I get good casts with it. Now, I'm thinking of upping the line size a bit and look for something in the 375-390 range to see how that works. As it stands, my buddy can cast the entire line and running line (that's how I know if a line and rod are a good match). I can get somewhere around 20 strips out + the head and tip. But MY preference is a lighter line. Mike Kinney, one of the guys who helped design the rod, suggests a heavier line. So I can't suggest which would work better for you per se. And then, after that you have to decide what kind of skagit line, compact or regular, which brand because not all lines are built the same as they have different characteristics such as front taper, rear taper, material used to make the line. Oy Vey!