This got me thinking. Back in the day one of the guys we flew with regularly would have his wife sit in the seat and call out when the fuel level was at the top of the vertical tube in the sight gauge. I always found this interesting as on my first Model IV, I fueled each tank a gallon at a time to get an accurate mark on gauge. What I found is that at the top of the vertical tube just as the fuel began filling the horizontal part there was 9 gallons in the tank. I suppose tom never flew with more than 9 gallons in each tank. I did the proportional fill both tail in three point and fuselage level as in flight. With a 90 degree barb, I suspect you will be covering more of the vertical tube - top and bottom for probably a visual indication only between about 4 and 8 gallons.

Regarding the three point and flight attitude readings. In the middle the readings were really close, but low fuel and high fuel indications were a gallon or two different. Can't remember high or low, but a gallon or two different.