How much twist you need in plying depends on how much twist you put in the singles ... and you can pick what you want your plied yarn to look like by how much twist you put in your singles.
Basically, singles with less twist needs less plying twist, and singles with more twist needs more plying twist. Mostly I aim for a balanced yarn, where the plying twist "balances" with the singles twist so the finished, washed skein doesn't twist on itself but hangs relaxed and open. The drawing shows two skeins, one under-plied and the other over-plied.
Before you let the singles wind onto the bobbin, hold the length of new yarn loosely so it can ply back on itself. This shows you what the balanced 2-ply would look like. If you are shooting for a 2-ply, then use the "ply back" test to ensure your singles have the twist you want for a nice 2-ply in the "ply back" test.
If you are shooting for a 3-ply, the singles need a little less twist -- a 3-ply at the same twists-per-inch as a 2-ply takes less twist out of each single (since it's taking it out of 3 singles, and the 2-ply's taking it out of 2 singles). So you can go for a "looser" looking result in the 2-ply "ply back" test and still get a nicely spun/plied 3-ply.
Of course, you should save a sample of the "ply back" test at some point in spinning the singles, to compare to as you spin the singles and also to compare to as you ply.
As you are plying, find a rhythm (number of treadles) that gives you the ply-twist you want and go with it; the "right count" will depend on your wheel's ratio of wheel to bobbin, the twist in the singles, and how much twist you're going to put in the ply.
(posted by me this day on Spin-List)