no, you don't have a variable cutoff, you just set it, and then have an on/off switch where you bypass the filter for normal/no portamento. it's a sim of a real analog portament. i'm not 100% on how it actually make it do the porta, i think it acts as a slew (although that might be the 'Rate' time based type ?).
and i think i found a value of 0 to work best. i would have to look at my synthedit file, that's what i did it in.
let's see if i can show it here/
pitch CV>>SwitchOn>>LPF>>>>>Osc PitchCV in SwitchOff>>>>>>>>>>Osc PitchCV in
so, whatever you need to do to code that in C++ (would love to see what that looks like btw, i know nothing about C++, really should learn a bit)