Hi,
To be honest, I don't have a clue.
My decisions in whether to supplement in potassium, magnesium and have extra salt were:
Salt - I'm getting night cramps in hot weather, and feeling floppy a lot of the time. let's give it a try - so I dissolved a stock cube in hot water (as per Volleck and Phinney's suggestion) and drank it. Felt better within about an hour. Nowadays, I just use a pinch of salt on my dinner, or have a bovril or stock. I aim for 2 g a day, but it is v approx.
Potassium - those night cramps - so I bought a pot of supplements (as suggested on this forum) and popped a pill at bedtime. Magically no cramps waking me up in the night. Only really need them in hot weather, but the chance of ODing on Potassium is miniscule, and I figure the optimum level is probably way above the level that prevents cramps! So I take one daily (when I remember). I was also having an odd little sore throat, most mornings. A tiny white kump/patch on one tonsil. One of the signs of severe pot' deficiency is mouth soreness and lesions, I think. Anyway, if I wake up with that, I think back, and have INVARIABLY been forgetting that pot' tablet.
Magnesium - suggested on the forum as good for LC, and not something the body can't eliminate with ease (if I recall correctly, it isn't something we store, and can easily wee out) so I took some, and noticed that my large muscles just felt 'easier' and more comfortable.
All very unscientific, but it works for me. And overall, I feel sooooooo much better.
In every case, I am confident that my supplements never take me above the Recommended Daily Amount.