I have a small VB program that calculate the inductance of an air core
coil.
Coil size must be expressed in millimeters ( length, diameter, turns).
Slides are provided to vary easily your input by 1% steps.

I can send you the exe if you want it. (8.10K)

As i recall, to get maximum inductance you must stick to this physical
proportion( length = diam/2).

Jean