I did the same thing with the Sportys universal stick.
Over a period of 2 months, I put the stick in prior to refueling to top, then noted the difference and calibrated it (slightly different aprroach to what Sporty said, but result should be the same). I found that the resulting curve is slightly non-linear, but that as an estimate, you may conclude that
1 Sporty line represents approx. 10 liters or 2.5 gallons.
The tabs are more or less there where they should be. The dipstick method naturally has some errors (no temperature compensation, indicated height depending on the the vertical speed your hand has while have putting the stick in, stick not exactly vertical, plane on uneven ground, etc.) But I would say that the comparison stick against refueling bill against Fuel gauges per tank is not more than +/- 1 gallon, which is acceptable.
Minimum measurement was in a nearly empty tank (3 gals remaining).
Caution: Don’t let the stick slip out of your hand falling into the tank!! I takes approx 1 hour to fish it out! (Now it has a selfmade collar bigger in diameter than the fuel cap).
Your 14 gals between tabs and max do not surprise me, this equals a hight difference of 4 mm or 0.15 inch only - pretty good for this crude measurement.
AND: if you can refuel only 14 gals from tab to max, then you must also redo the dip stick calibration process. It took me more than 20 repeating attempts to finally get a calibration with an acceptable error envelope.
I hope that everybody flies with at least 45 - 60 mins reserve!!
Timm Preusser N747TG
Just had the unique instance of having both tanks depleted right to the tabs - exactly (checked it out with a flashlight).
Measured with my dip stick, and compared to gauges.
Gauges said 14 gal in left, 16 gal in right, but right is always 2 gal high.
Dip stick, using my calibrations said about 15 on each side.
All of the above, as we all know, is possibly representative of the inaccuracies of the gauges and certainly my self-calibrated dip stick.
However, I supervised the filling, and it went all the way to the top, and each tank only took 14.0 gallons.
Any other experience on this? I’m wondering if the tabs are off by a gallon.