Assuming the simplest model (probably wrong), the antenna height
shouldnt matter cuz VHF is s'posed to be line of sight but assume it
is. So you use 1/r^2 and get that 1 watt should reach 1/5 as far as 25
watts. This is prob close enough but I bet there's some refraction and
bending of the pattern 'n such along the earths curve that might depend
on total power (I am not sure how it would depend on power though).