Stacked Primary

["Steve Young" <youngs-at-konnections-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

I am thinking about building a stacked flat spiral primary to cut down the
diameter some.  I need a sanity check and some ideas on mutual inductance
effects.
 
A "normal" flat spiral primary for my design would have an inside diameter of
7.2 inches, be wound with 1/4 inch tubing spaced 1/4 inch apart, and be about
12.4 turns which ends up 19.1 inch diameter.  According to Wintesla, the
inductance will be .0563 mH.
 
Now, if I have two flat spirals in series, each half the above inductance, each
coil will be 8.9 turns with outside diameter of 15.6".  This is better because
it is more compact.
 
If the two coils are stacked, with 3/8" acrylic in between for insulation and
support,  Then the total inductance will go up by some factor due to mutual
inductance.  This means I could reduce the turns by some factor to end up with
the same inductance.
 
Can someone calculate the approximate number of turns my stacked primary will
need to have about 0.56 mH?  
 
I will probably make a temporary one with #14 wire first to measure the actual
inductance, before I build one from tubing.
 
Thanks,
--Steve