it is still best to check it with the pump, i.e.
Mpguino calibration - EcoModder
It is just like a car, keep everything the same while calibrating (same pump, same side, etc) except that you can see the gas through the gascap and so you can find some mark (possibly the bottom of the lip of the tank) that you call the refill line and use that. I would fill it on the sidestand so there is some air left in the tank if using the lip.
There's plenty of bikes in the garage (including mine) that get consistent figures with no other instrumentation besides the gas pump and the odometer, so it is doable.
I may have had an easier time though, I put a 5 gallon tank on my bike so the margin of error at refill is small.
You could do something similar to reduce the margin of error, get a 5 gallon gas can, fill it precisely using the fuel pump to 5 gallons, then go fill up to the lip, and only refuel from the gas can during calibration until the gas can is completely empty (get it all out and in the bike) but don't reset the guino, then when the last tank is getting low, go refill like normal but add 5 gallons to your calculations for pump gallons.