If you can't be bothered with fitting this extra stuff [coolant recovery system] under the bonnet, here's what I do.

1. Initially, top the standard radiator right up.
2. Run it up to temperature once or twice to ensure the system is fully 'overflowed' - so rad now at its optimum level.
3. Leave to cool down.
4. Get a squeeze bottle (with a tube added if necessary) and suck out all the water you can see in the MGA's radiator neck.
5. Measure the quantity, mark the bottle at that point, and keep it!
6. Put it all back in and refit cap.

To check level any time after this (when cold) suck out all you can see as before. Compare this quantity with the original measurement. If less top up with your A/F mixture to that original measurement and pour it back into radiator, and out the cap back on. You are now back to that ellusive optimum level again. You have to check the oil and brake fluid levels anyway, so checking the radiator is all part of our special little ritual before a run in an MGA.