Radiator overflow bottle

[Roger's E Type]

My 1966 Series 1 4.2 is fitted with a larger capacity radiator (for air con) and an overflow bottle. I have been told by a mechanic that the bottle is too low and therefore not effective. Does anyone know what the relative level of the bottle to the radiator should be, ie should both fillers be at the same height.
FYI the bottle is connected to the overflow pipe of the radiator by a rubber hose that goes into the overflow bottle at the bottom - there is also an overflow tube for the bottle up near the filler.
Cheers, Roger