If it works, then that is enough. I don't put alot in the tub either.
A cup is 8 ounces. I use about 1/2 cup in a bathtub for soaking.
I did make compresses for my knee when it was acting up on vacation. We don't have a bathtub up there. So I put about 1/3 cup of epsom salts in a dishpan sized tub and made compresses and put them on the knee. I expected the cloth to absorb alot of it so I made those more concentrated. I did this for about an hour on the dock while taking the sun.
I tend to be rather conservative most times.
You can always add one ounce more the next time if you feel you had no relief or improvement. But I wouldn't go past 1 cup --8oz-- at a treatment.
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Weezie looking at petunias 8.25.2017
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