You could try the forum search feature located on the "MENU" block top left of the page. The minimum length of the roundhouse track would be what's needed to clear your longest loco/loco & tender from the TT. I don't recall ever seeing a roundhouse sitting right on the edge of a turntable, but it could be done that way I suppose. Again, the TT should be large enough to handle your largest motive power. Your roundhouse side tracks need to line up with your roundhouse door openings. It would be nice to have your TT lead in long enough to hold your largest engine.
I would eyeball some examples.
Simon
