Determining the layout spacing of a turntable and roundhouse can be a bit of a challenge as you plan your layout. In order to try to help provide some guidelines and examples, Millhouse River Studios,and Korber Models have worked together to come up with a Roundhouse and Turntable space planing guide.
You can download this guide at www.korbermodels.com and go to the services and links page to get your copy of this useful document.
While every installation of a Turntable and Roundhouse require a bit of adjustment for the particulars of your layout, CSXAL and I hope you find this as a helpful start.
Regards