Basics
Answer to the original question: D is about 1,06/100 of your speed, of course same amounts, knts, kms, meters/day, what ever... when the turn is basic = 3 dec/1 sec.
r is half of the D = 0,56/100
All this is very basic maths you must know before you can get PL, at least in Finland. Travelling speed X your turn takes 2 mins, so you travel during the turn X/30, the D is then X/30x3,14 and r is half of that.
So, using 1 % of your speed you get the diameter of your turn. If you like to be more accurate, you can add to the result 6 % and you are quite near of the absolute truth.