The Duncan Tone Stack calc is for PC only, I'm on a Mac so I use the online TSC (link was in my post and in BtR's above.
If you look at the screenshots, you'll see at the top of them it not only does Muff, but all sorts of tone-stacks including the SWTC (Stupidly Wonderful Tone Control) [Edit: MARK HAMMER's, which has been tweaked by AMZ's Jack Orman], which is a favourite of mine, but I only just noticed it on there while typing this!
The only thing I'd change is to make the Muff Tone-Stack on the TSC look like a Muff tone stack...
< HPF vs LPF MUFF-TONE DIAGRAM BY CODA EFFECTS
I mean, c'mon TSC people! My brain always has to translate the TSC's schematic to the Coda example used by just about every Muff schematic ever (with a stock tone-stack at least)...
ENNEWAYZ...yeah, TSC, very cool resource. It was ages before I realised I could adjust the fader lower left — and the graph responds to where you've set the TONE fader/knob. Change it to Audio-taper (logarithmic) and all that.
Have some fun with it, compare a Purple Ram's head tone-stack's scoop to that of a black Russian — etc.