[Lowfer] variometer design questions

[Eric Smith]

My new coil is going to be quite large and wound to about 7.8mH in the DEX 
style with taps at the cold end.  I expect to be tapped up from the cold 
end a little as I am now on my small, temporary coil of about the same L.

 * How much L should the variometer have?  Is it usually a certain 
percentage of the coil, or just the amount you need to provide a given 
delta?  For example, I think Mitch's variometer is about 500mH and his 
primary is about 2 or 2.5mH.  Since my primary is so much bigger (7.8mH) 
does my variometer need to be 1.5mH or more, or would 500 be enough for me 
as well since I'll be tapping at some point near resonance and then just 
using the variometer to tune above or below that _slightly_, and not 1.5mH 
or more?

 * Physically how should it compare to loading coil size?

 * My loading coil is a basket weave style with plywood top and bottom and 
hardwood dowel spokes.  What about a small version of this for the 
variometer instead of a single layer coil?  I could use thinner plywood 
ans smaller spokes.  Would this be better than the single layer coil on a 
form such as pvc pipe or whatever?

Thanks,
Eric