How about using the one / sixty rule.

For every degree of track you drift 1 mile per 60 miles.

So if you fly perpendicular to the radial you need
sixty times the time you need for one radial to the VOR.

One radial is half a dot.

So you need 30 times the time you need for one dot
to the VOR, or 10 times the time for 3 dots.

(I wouldn't calculate on half a dot results, after
having done some VOR-Flying in real world).