Two USAF rules of thumb (we're not any better than a thumb):
Mach No. squared, i.e. M.80= turn radius of 6.4 nm
Groundspeed (in nm per minute)- 2 i.e. 360 Knot GS = 6 nm/min = 4 nm turn radius.
There are others for the mathematically inclined (not me), if you need more accuracy than that you are not flying, you are computing.
