I tested with 'zinc test' (zinc sulphate liquid)again last night, i was borderline deficient, so need to load up again. I've been off zinc supps for 3 weeks whilst on holiday though. Ive no white spots.
However, It occurred to me that perhaps i should be swallowing the 'zinc test' after testing, though the bottle doesnt specify this!
My bottle also said that i should discard the contents 7 days after opening though, which sounds crazy, its been open for 2 months, may affect the results.