That will go on a P/P volume pot on/off switch.
No tone pot, just a cap wired to the switch, either engaged or not.
Something that would give me the sound of a tone pot on say 7, just rounding over the top end.
.01uF? .002uF? .003uF?
What's the problem with just turning your tone pot to 7 to get the sound of a tone pot turned to 7? Does your guitar not already have a tone pot...only a vol pot? Why have a p/p on the vol pot to act as an on/off? Just turn the vol all the way down to "turn it off". Then you could add a 330k resistor and a .022uf cap to one end of the p/p. Those values are a rough approximation, but you could do some measurements and calculations to come up with the exact values that would work for you.