Hi Rob,
I see. Okay, here goes.
All you valve and fluid guys out there, please take a look at this - cause I may be off of my rocker.
Before I begin, are you allowed to vent natural gas to atmosphere?
Anyway, here goes.
Assuming that all of your wells are pretty much the same, and that you don't have a lot of water coming up with your gas (like, almost none at all), the flow rate can be calculated from knowing the Cv of the valve at a particular opening, pressure drop (which is from your pressure gauge to atmosphere), the density (or specific gravity) of the gas (you need to analyze the gas, and why I said assuming the wells are pretty much the same), by plugging them into the Cv equation (available in most handbooks).
The tricky part is the +/- 5-7%.
I am not sure how you would determine exact valve position to correspond with the vendor's Cv values at 3 positions. If you are even a few millimeters off on the valve position, depending on where you are on the curve, the Cv change may be a % or more.
The density of the gas would also be difficult. It would vary in the same well.
Have you though about a portable flow meter?
If you are indeed allowed to vent to atmosphere, a pipe with flanges and flow meter (clamp-on ultrasonic flow meters, turbine, vortex) would be a practical, portable, with good turndown, and be a fairly accurate way to measure flow. Of course, you would need power. Hey, is this the driving concern - you don't have power at these wells? If so, I guess you would need to buy some batteries and step up the voltage to power the flow meter's electronics.
Like I said. I may be waaaaaayyyyyyyy off my rocker. It's getting late in the afternoon.