Did you build the layout above? Is it verified?
Anyway, if you want to add a gain pot you could make it in two ways:
- Lug 3 of a 1k linear pot at C stripe, left side.
- Move the C1 cap negative side on the D stripe and make a cut track between the two lead.
- Lug 2 of the 1k linear pot at D stripe.
- Input on the lug 3 of a 500k linear pot.
- Lug 2 of the 500k linear pot at F stripe.
- Lug 1 at the ground (E stripe), or if you can find a way, through a resistor, to set the min gain setting, then to the ground.
- If you notice treble loss at min gain setting, try to put between lug 2 and 3 a small cap ceramic cap, I don't know... 50pF, 100pF...
If you want you can add both the pots, I'm almost sure those will work in a very different way.
I build pedals