Bloviator mods questions...

[CheapChip]

Hi,

I'm planning to build a Bloviator soon, and I'd like to mod it with ideas I found about Bajaman's Sonic Stump as Bloviator is based on this project.
All mods are shown within this topic on FSB:
http://www.freestompboxes.org/viewtopic.php?f=13&t=395&start=80
But I have a question about putting switches and caps for C4 and C5:
How can I determine values of those switched caps if want to control specific frequencies.
For example: I think i'll use it with a bass guitar (with input cap "bass mod") and I'd like to tweak on 110hz, 220hz, or such frequencies with "Contour" pot. How can I find the correct cap value?
2nd thing: I'd like to make the same thing with the "Mid Pot mod" (replacing R16 with a 10k resistor + a 50k pot). Which cap have I to replace with a switch + caps?

Many thanks!