[Amps] 3CX800 stray C to chassis?

[Conrad PA5Y]

As I do not (yet) own a decent LCR meter does anyone have a decent ball 
park figure for the stray C to chassis for a 3CX800A7? The side walls 
are 1.25 inches away from the tube and the lid is 2.5 inches above.

Also thanks for all the help with my previous questions. I should 
mention that this is a retrofit to an existing 2m amp and C1 is 6pF 
maximum and I would like to use this part if at all possible.

I have a design that works on paper for strays C of 4pF with a Q of 8 
using an L-Pi-L configuration as recommended by a few of you. The 4pF 
stray value does not seem unreasonable but it is a guess. I am sure that 
someone must have measured this.

Regards

Conrad PA5Y