As I was finishing up yesterday, I realized that the roundhouse needed to come closer to the turntable a little bit so as to avoid sharp points at the pit edge.
A quick review of Engine Houses & Turntables on Canadian Railways, 1850-1950 by Edward Forbes Bush (thanks for the loan, Scott), shows that a very common format at least on Canadian railways was for adjacent rails to meet or nearly meet at the edge of the pit. There they were bolted together, probably with a block. The feet would have had to be planed, but we’re at least not talking about a full crossing of the rails. That seems to have been very rare in Canada, and the book shows only one illustration where house tracks crossed over one another at the pit wall.
Making the assumption that Pembroke followed the common pattern, we come up with a roundhouse that looks pretty good, sitting 71′ from the pit wall. What’s more, it looks like all the tracks fully fit on the layout, and almost all the roundhouse fits too.