Dave,
I have attached one page from a very old copy of Track Planning for Realistic Operation by John Armstrong. That page appears to use uniform sized turntables to show the relative positioning (and space use) of roundhouses based on the degree of angle used by each stall in each roundhouse. Unfortunately, the discussion doesn't provide any real dimensions, but it seems clear that the amount of space used by a RH & TT combo can be dramatically impacted by the degrees of angle used per stall.
I also vaguely remember a discussion in a RH & TT construction series of articles in Model Railroader magazine back in the 1960s or 1970s. If you can find them, they might also provide some useful planning info. Hope this helps a little bit in resolving your dilemma.
Chuck
