I never dry fit the pipe. I know alot of people do, but I do not find it helps. I typically measure the depth of the hub (actually know most of the depths by heart) then I use the depths plus the hub to hub length (or center to center length) to make all my calculations. Once I have the length, I actually put them together with the glue. Then I build my next section. The reason dry fitting is difficult is, the inside of the fitting is tapered so that as you insert the pipe when you are gluing it, it gets tighter. When the pipe is inserted all the way into the hub, there is virtually no separation between the surface of the pipe and the inside surface of the hub. But at the outlet of the hub, there is a small distance between the pipe and the surface of the hub. This gap is filled with glue.