You may wish to do as I did when I first got my T'Hawk. I purchased one of the clear hollow plastic fuel gauges (the universal model) and progressively logged the levels on the chart that comes with the gauge, as I filled the tanks from empty.
I commenced filling each tank until the fuel at the bottom of the tank had crept up to the point where it was directly below the fuel filler cap and noted how much fuel had been dispensed at this point. I then continued filling the tank in 10ltr stages, marking off on the chart the position the fuel in gauge in relation to the graduations marked on it.
All done on a level surface and extremely time consuming but when completed, an accurate method of determining the fuel quantities in the tanks.
Of course, you could transfer the readings from the chart directly onto the gauge but make sure whatever method you use, the fuel doesn't remove the figures you mark on it. The method I used was to engrave the figures into the gauge and rub some black paint which is not affected by fuel into the engraved marks.
Hope this is of some use.