That is a good amount, but a standard blood test will not reveal the cellular amount of potassium or magnesium. They are intracellular minerals.
To cover the minimum RDA of potassium from bananas; 1-2 kilos a day..
The levels in blood are kept steady, as it is so important for the body that it steal these two if necessary; from bones and tissues.

Potassium and magnesium are found mostly inside the cells (only 2 % of the magnesium in the blood, similar for potassium; and as I said; the levels are kept constant) That means that however short one are, this does not show up on a standard blood test.
Red blood cell test, full cell test etc is the way to go for testing (expensive!), but I prefer to look at symptoms and adjust from there.

http://www.stopthethyroidmadness.com/potassium/