Ed,

I've always used a teaspoonful of Epsom salts to a half litre of water in
one of those handheld sprayer bottles for a foliar spray and have never had
problems.

However, I've just checked the RHS gardening book and it gives 210 gms to 10
litres. As I do not have a sprayer bottle that big (!) I weighed a spoonful
of the stuff I use (from a chemist's) it weighed about 5 grammes therefore
I'm under applying it to my plants but that's better than too much.

Regards

Geoff