First of all, the FQ does what it said on the tin. However for my purposes it would be better it it was a little more accurate in the lower ranges.
However after a bit of fiddling and more fiddling it turns out that if you take the formula of the trendline when you plot target vs measured cfm you invariably get a garden variety 1st order equation. This is good, as this keep things simple.
So you plot the points and excel spits out the trendline and formula Y=Ax+B where aX is the slope and b the intercept.
So you can either fiddle with calibration dial until you get an Ax value as near to unity (1) as possible so that you basically ignore it so that you only have to deduct the value B from all measured values . ( In this case I got it down to y=1,009x+6,3121). This will get you very reasonable results to within 2 % over the entire range
The other option is to just adjust it to say 100 cfm (middle plate) measure the other two and not ignore the X value of the trendline. Just to see if this works in general I tried it using a lower depression ( 10 inch actual >28 cfm) which is pushing the correction a quite a bit as well.
y = 0,9819x + 11,282
R² = 1
rest is rather simple : take the values and multiply by 1,0181 and deduct 11,282
this is what you get without correction, the values are so far off as to be quite useless for anything other than comparing between your own tests.
and after correction
in percentages error (the number of significant digits is of course less).. just read it as about 0.2-1%
So all in all pretty impressive for a piece of kit that does not cost all that much. Hopefully Audietech makes a simple addition so you can basically input the trend-line so that you do not need process the data separately.
It already has a % adjustment and leakage built in, but the latter does not seem to work like I think it should otherwise that would do exactly the same.
Audie tech send an update to fix the issue I had with the CFM leakage not working properly (the customer service is really good, especially considering that my unit is from 2007).