Most people will tell you the only reliable sensor on the grey tank is water backing up into the shower. That did not happen so you were not completely full. This question came up a few days ago and someone had a good suggestion.
Completely empty your grey tank. Make sure guage reads E. Dump 1 gallon in at a time until it reads 1/3 and write how many gallons. Keep adding until 2/3 full and note how many gallons. Keep adding until it says full and write how many gallons. Keep adding until water backs up in tub and write how many gallons. You will end up with a chart
Empty = 1 - 4 gallons
1/3 = 5 - 10 gallons
2/3 = 11 - 15 gallons
Full = 15 - 20 gallons
That will give you a better idea from any marker on the guage how much room you have before you hit full.
If you have not yet replaced your shower head with an Oxygenics that will also help save water. The flow rate is about the same as the factory shower head but the water pressure difference is night and day difference. You will be able to rinse the soap from your hair with much less water.
Our gray tank is around the same size as yours. This 3 night weekend I took 3 showers, wife took 2 showers, and she washed both of our boys twice. We did not fill that grey tank. We take navy showers as quickly as posssible but I don't think we go crazy with it. Sounds like you follow the same process as us. I never checked the guage. Where we camp they do not have water hook-up so our 38 gallon fresh tank has to get us through our trip. No chance of us filling the grey tank and also using the toilet before running out of fresh water.