Can you use a clip on line counter to measure casting distance on a round baitcaster and Is this the best way to do it?

Indirectly I suppose. You would have to cast without it on, then install it, and reel in to get a number. I think they are meant for vertical fishing. At least that is my guess, Ive never used one. Or just go cast in a big grassy area and step it off to get an idea how far you are casting.

I don't think a line counter for casting is as good as just being a good judge of distance. The way I understand it, line counters are good for vertical fishing in deepish water, or trolling where it allows you to know how much line you have out so you can judge the depth your lure is swimming at behind the boat.

Head to the local park. Measure 100yards. Stretch the line and mark the line at 100 yards with a Sharpie. Mark it at every 20, 30, 50 using different colors of Sharpie. Or different lengths of the same color marks.

Fill the spool after the line has been marked for distance.