Hi, Nap.
I do not know the exact diameter of the crown of the officer's peaked cap of that period. They at least differed in the size of the peaked cap itself, besides, depending on the factory that produced the peaked caps, as well as the year when it was sewn, the shape and size of the crown could be different. In addition, officers in their everydaily uniform often used peaked caps made to order in the studio or directly in the military unit.
I made the size and shape of this peaked cap according to the consultations of the customer - he is a former officer, and served in the army at that time.
As far as I know, in the 80s in the Soviet army were caps sample 1973. Below is a photo of some of these caps of different years of release with dimensions in centimeters.
1973.