Filter key tracking..

Hey everyone :)

I've been looking into key tracking for a filter i'm designing in a monophonic subtractive synth. I have found some info and i kind of understand what it does but how do i implement this in max/msp?

So far i have a check box to turn key tracking on and when its on the cut-off of the filter is the frequency of the midi not being played but that's not right is it?

There must be a bit of maths involved but i can't find anything online to help me. Could someone please point me in the right direction? :)

Paul..