Just a thought...
GHL computes the milliliters volume of their intake tube based on a 4mm internal diameter tube. You tell it the length of your tube and it converts that to a "ML" volume - and then advise you to just add 7ml to your total calculation to allow for the GHL filter volume.
Perhaps, if the RC filter is larger/smaller than 4mm internally then the "sample tube ml" amount would need to be adjusted?
I'm gonna try calibrating with actual fluid in the tube tomorrow (too dang tired from working all of this today), but I'm using all of the stuff that GHL provided, so it should be correct.