Need help with John K's mid cut filter

[Synsound]

I'm posting this here as the original thread at the layout page seems dead.
I have built a Fender Blender and it works great except in treble boost mode it's all heavy miss. John K. suggested changing one of the 47k resistors to 10K (not sure which)and using this filter:



It says to use the calculator at the link which is here:
http://www.geocities.jp/dgb_studio/hart_calc_e.htm
I'm not sure what I'm doing and what frequencies would provide a general mid cut that would work. The variables are confusing me.
I am also a bit unsure of where to tie this into the main board. Here is the effect layout. Any help would be appreciated.