Tube Coil Calculation

I am curious about establishing the value of the primary capacitor for a tube
coil. I am aware that there is a simple formula for picking the optimal tank
cap for a standard disruptive coil based on the power of the transformer
feeding the tank circuit.

Brent Turner's book suggests that this is not a good idea for tube coils and
that the tank capacitance should be kept quite small.  He suggests .0015 to
.004 F for the primary tank cap.  But I really cannot find any guide lines to
picking the "right" capacitance.  I've looked at several schematics for tube
coils using various power levels and tube configurations, but cannot seem to
establish any sort of rule from these for a tank cap.

Anyone have any experience with this?

David Hutchison