Most useful for crossover inductors are multilayer coils. There are several methods for calculating, but the Wheeler formulas are said to be the most accurate. I have a formula I believe came from the ARRL handbook:
Inductance(in microhenries)=0.8*N^2*R'^2/(6*R'+9*L+10*Dr)
N is the number of turns
R' is the average winding radius
L is the axial length
Dr is the thickness of the windings above the core = (Outer diameter - core diameter)/2
I believe wheeler said the formula is accurate to ~1-2% if the terms in the denominator are about equal.
By searching for Wheeler formulas I found the following link, which has a nice write-up with references:
http://home.earthlink.net/~jimlux/hv/wheeler.htm
Unwinding inductors
Inductance varies by the number of turns squared.
L2/L1 = (N2/N1)^2
Count the original # of turns (N1) and use initial inductance (L1) and Desired inductance, (L2)
Solve for N2:
N2=N1*sqrt(L2/L1)
N1-N2 = number of turns to remove.