Coupling constant for 4.25" secondary

Hello All:

One of the few things I have yet to optimize on my coil is the pri/sec
coupling, though from what I've read, it can make a big difference in
performance.  I've not had problems with racing sparks on the secondary.

My coil uses a 4.25" dia x 23.5" long 22AWG secondary and a 15 degree
conical .25" copper tubing primary, with .25" spaces between turns.  The
innermost point of the primary is at the same vertical level as the first
turn of the secondary and (only!) 7/8" horizontally distant.

(complete specs & pix at http://people.ne.mediaone-dot-net/lau/tesla/tesla.htm)

I've measures the coupling coefficient "k" two different ways with
identical results:  0.139.  I just tried raising the primary up by 0.25"
and that changes it to 0.144, but it doesn't seem that the start of the
primary should be above the start of the secondary.  

I know that there are so-called magic values of k that make for the
cleanest quench, the nearest one to my coil being 0.153.  

What I would like to ask is, for other coilers using 4.25" secondaries,
what values of k have you achieved?

Gary Lau
Waltham, MA USA