If you measure the fuel level with a dipstick on the C172 I fly, you can get two different readings. This is because directly under the filler cap opening there is a cuboid-shaped sensor. Obviously, measured from the top of the sensor, you get a much smaller value than if you lower the dipstick to the side of the sensor. (4 USGal difference) Which value is correct?
The bottom of the tank is flat so you want to make sure the dipstick is touching the bottom. In any case, if you want accurate readings, it's not sufficient to just dip the fuel and estimate the level between the bottom and top of the opening, because if the tank is sloping, or is tapered, or is irregular, it doesn't give you an accurate reading.
Absent a commercial dipstick that is custom graduated for that tank, your best bet is to mark off your own dipstick by draining the tank through the gascolator, then filling it in 1 or 2 gallon increments and marking off the levels yourself. I've always done that with the airplanes I've flown regularly.