the cap should be the highest point in the system. you can use a pipe filled with some water if the measurement is too fine or too awkward to identify. By looking at it, the top edge of the top radiator pipe should sit lower than the radiator cap. If your situation presents itself that the radiator sits lower than the engine like in a later mazda RX7 or RX8 with the radiator slanted, then you need to use a high mount bottle with pressure cap like the RX7, Subaru boxer engine car, MB, VW/Audi has.