The approximate length of the race rail is pi*d. So, for a 70" diameter turntable, (the race rail is not quite at the extremity so the diameter of the race rail will be slightly less) the circumference is pi*70' on the prototype. For a 7mm scale model, this becomes pi*70*12(inches)*25.4(inches to mm)/43.5(scaled). That's 1540mm. With a chair every 4deg as you propose, that's 17.11mm chair centres not 5.45mm!
Dave