base turn
Don't have a quick rule-of-thumb but a circle is almost 3 times the diameter.
At 200 kts the radius of turn for 25 degree of bank is 1.25 nm so the diameter is 2.5 nm so the entire circle would about 7.5 nm so half of that would be 3.75 nm. It is actually 3.9 nm.
So the turn from downwind leg to final at 200 kias and 25 deg of bank is about 4 nm.
Formula for radius of turn in feet:
TAS squared divided by{(11.26) X (tangent of bank angle)}
Need a sientific calc to help.
eg : TAS 100 kts, bank angle 15 degrees
TAN of 15 = 0.267949 times11.26 =3.0171. TAS 100 X100 =10000 divided by 3.0171 = 3314 feet or .54 nm.
Half the circumference (base turn) is .54 times pie ( 3.1415) = 1.7 nm.
Hope that helps.
Thermostat.