Just because you have a "FULL" mark doesn't mean you fill your
oil up to that mark. Your optimum fill level might be halfway between the "ADD" mark and the "FULL" mark, but that only really applies to a dipstick that is properly calibrated and when your
oil pan is level. That said, do you know what brand and model your
oil pan is? If you do, then what I would suggest is draining your pan, level your oil pan by jacking up the back of the car (or the front) until your pan is level, then pour in one quart less than the pan capacity (which is usually one quart less than the manufacturer's
system capacity number), put your dipstick in and make a mark for ADD at the shown amount, then add another quart and mark that FULL. Then drop you car back down and see how the cant of your engine affects your readings. I keep my oil mid way between ADD and FULL at horizontal and that results in significantly less oil usage than if I fill it to the FULL mark at level, and filling it to FULL mark with the engine tilted results in a significant overfill condition.