You have to use one or the other. If you have it set to odometer setting, you put the reading in every time. You can switch to trip meter reading in the settings, then you put the miles per tank instead. Does that help?
I think you can switch between the two methods, but best pick one and stick to it, reduces the likelihood of an error occurring. Take note of figures just incase though.
It would be better to be able to enter both and entering one would automatically update the other one like on spritmonitor. This way you know what is your car's mileage. Even Acar allows this ...