Why can't I Dry-fit PVC fittings?

For those situations where dry-fitting is REALLY important, you could cut a slot lengthwise in a sacrificial piece of PVC on a tablesaw. That would allow the pipe's diameter to shrink as it's placed into the fitting and bottom out properly. Once the assembly is complete, mark all the pieces, take it apart, and duplicate the slotted pieces in new stock. A typical 1/8" kerf would allow the diameter to shrink by .04" (the shrink is independent of the original diameter) -- that should provide enough slop to dry-fit, and the springback should hold things in place fairly firmly; maybe too firmly. Another opportunity to experiment. Hang on to the slotted stock to use the next time.

This is getting a lot more complicated than it has to be. Just make your measurement, however you do it. Then calculate what the length will be to the "FACE" of the connection, and finally add the depth of the socket.

Gosh, you take all the fun out of it.

Dry-fit with moderate force, mark a line on pipe @ mouth of fitting. Then pull it out, & add depth of socket (eg 1.5") to your mark for full-depth measurement.

I haven't actually done this; I just like being a theoretical plumber (I'm an amateur really). I usually either hold the pipe/fittings next to each other & scribe, or use a tape measure.