It all depends on what units you use for V and for g.
If you use knots and NM per squared hour you get radius in nm.
If you use meters per second and 9.8 meters per squared second, the radius is in meters.
9.8 m/s^2 = 9.8 x 3600 x 3600 / 1852 = 68578.83369 NM per squared hour
For a 250 kt with a 25º bank angle:
r= 250^2/68578.83369 x tan 25º= 1.954 NM
If you use NM/min, g = 9.8 x 3600 / 1852 = 19.04967603 NM/min^2
tan 25º= 0.466307658
tan 30º= 0.577350269
g x tan 25º = 8.883009818
g x tan 30º = 10.99833558
So if you find your speed (GS) in NM/min you can use, as a rule of thumb, that speed squared and divided by ten. This figure is approximately your radius of turn for a typical 25º-30º bank angle turn.
examples:
180 kt, 3 nm/min, 0.9 nm radius
300 kt, 5 nm/min, 2.5 nm radius
420 kt, 7 nm/min, 4.9 nm radius
I use it to join, or exit, DME arcs and it works really well. It is useful too when you have to initiate a turn to intercept a radial, or to intercept the LOC if you have distance and a bearing pointer. Sometimes if you wait until the CDI moves it is too late already. No need for the FMS!