Circumference of a circle = 2Πr.
r = (Half Circumference) / Π.
Rate 1 turn... 360° turn takes 2 minutes... 180° takes 1 minute. so...
Ground speed in NM/Min. 180 kts --> 3NM/min.
So in 1 minute you cover 3 NM and in this example that's half the circumference. So if you assume Π ≈ 3, your radius become ≈ 1 NM.
Ground speed in NM/Min. 120 kts --> 2NM/min.
So in 1 minute you cover 2 NM and in this example that's half the circumference. So if you assume Π ≈ 3, your radius become ≈ .66 NM.
Ground speed in NM/Min. 210 kts --> 3.5NM/min.
So in 1 minute you cover 3.5 NM and in this example that's half the circumference. So if you assume Π ≈ 3, your radius become ≈ 1.15 NM.