You should be able to calculate the difference between any two positions on the quick change chart and apply that to any other known pitch system.
For example between positions 7 and 9, you'll cut 12 or 14 tpi in Imperial units. The ratio is .85714
So now, if you knew what pitch you got in metric units at position 7, then multiply that by .85714 to get the pitch when set at lever position 9, in this case it works out to .42857 mm.
If you have a dial indicator, you could accurately measure the pitch of the thread by screwing the adapter 1 turn into or out of the body and seeing how much rise you get. Or measure across the assembly with your caliper and unscrew the fitting one turn and measure again. You should be able to improve on the 30% error you've got so far
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)