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.