Sometimes a picture is worth a thousand words so, picking up on Paul's idea, here are a R1 semicircle (inner), a R2 semicircle (outer) and, in between them, a semicircle made up of two R2 (1500) curves at the ends and four R1 (1100) curves in the centre . . .
Whether that fits your platform depends on its width (I'm surprised that no-one has asked?)
Cheers
David