When my '84 van is turning 4000 rpm in 4th, I'm going a checked GPS checked
speed of 75 mph.
Looking at your formula it appears right, but I question the wheel radius at
12 inches.
Mine works out bigger.    I am using 205/70 x 14.

205 x .7 = 143.5 / 25.4 = 5.64"

(5.64 x 2) + 14 = 25.28" diameter which gives a radius of  12.64"

Hope this helps,   Steve