Expert advice needed! Theremin coil question

[charbot]

 I want to add an inductance coil in series with the pitch antenna to get better pitch linearity, like this guy did to his theremax: 
link:http://www.thereminworld.com/gal-pom.asp  (scroll down about half way).

How do I figure out how what size coil I will need?    this site seemed like it might help : http://www.oldtemecula.com/theremin/observations.htm ,but I couldnt find the info i need.
Is there an equation (or series of equations) that I can use?   I think that If I found out how to calculate the  appropriate coil value for my theremin, I could definitely  figure how to make the actual coil.   Ive found a couple sites that cover hand winding/ homemade coils. Been wanting to try it out.
    my theremin Uses four P-C70-OSC standard oscillator internal tunable coils and   the pitch operates at about 750khz.
Here is what I've built: http://www.thereminworld.com/files/Projects/Forbes/theremin.gif
I posted a build report a few days ago: http://www.diystompboxes.com/smfforum/index.php?topic=56753.0
... Anyway, any help is appreciated.  Thanks a lot.

[charbot]

UPDATE:  I gave "Lev's Antenna" another try this afternoon. http://www.oldtemecula.com/theremin/ult_antenna/index.htm   Good news, I got it to work. I must have been impatient with the fine tuning last time I tried it.  I cant say that it provides perfect linear pitch response, but after fooling with it for awhile I noticed the spacing between notes to be more evenly spaced. the AM radio tuning cap is used to spread out or crunch up the distance between notes , adjusting the instrument's sensitivity to the player's hand.
Its better but, alas ...It looks totally DIY , PVC antenna ,coil is a  spring from lowes, etc. ..which is cool and all, but Id still like to find a more traditional way to achieve this  so If anyone  can chime in, or share some knowledge, It would be a big help.
   I cant imaging that the coil  has to be that accurate.  Lev's Antenna is a just  Door Spring.  It seems that the tuning cap can make up for a lot.