The formula was indeen an approx. It is not bad for say #20 wire.
If one wants more precision one has to consider where the nearest
ground plane is. One can calculate the inductance of a straight wire
h mm above a ground plane as
-
L = o.0004605 * b * (log base 10 (2h/(a))(b+sqr(b^2=a^2)/(b+sqr(b^2+4h^2))))+
0.0002(sqr(b^2=4h^2) - (b^2+2^2)^0.5 = b/4 - 2h +a)
-
where l is in microH a = wire radius in mm, b = length in mm.