I use two methods of fueling. If a short flight just in the immediate area, (An hour or less), I'll top off the right tank. Giving me at least 1.5 hours of fuel minimum. What remains in the left tank is just added buffer.
If flying cross country, both tanks are topped off. Three hours of fuel minimum. Reality being closer to 3.5 hours. Cross country flights are planned for 2.5 hours or less. Actually, closer to two hours. I am ready to strecth the legs by then.
So either one, or both, tank(s) are full when I take off. No need to physically measure what is actually in the tanks! Simple! Foolproof! Why fly with any less??