You have to find the perimeter first, then divide it by the total number of rotations, then set that as the value of forward in your loop.
E.g.
make "d 100
make "p 3.141592654*:d
make "i :p/36
repeat 36[fd :i rt 10]
you can have all of them in a single statement as
repeat 36[fd 3.141592654*:d/36 rt 10]
where d is the accepted value of the diameter
solved Program to print a circle of given diameter