I never use 2 cups!

I use about a Dixie cup amount in the tub.

I have also made compresses with a very concentrated solution on vacation. We don't have a tub or running water there.
So I got a dishpan and put about a Dixie cup of epsom salts in it and used
a thin cloth (I think it was a HandyWipe --you know those towels you buy to clean with --they don't have any chemicals in them. They have holes in them tho.) I folded it over for several layers.

I'd sit on the dock and refresh it every 10 minutes or so. For about an hour a day. I did this for my bad knee which was terrible at the time. The water was never really warm much and that didn't seem to matter...the temp ...since it was only compresses. The epsom salts worked really well that way too.

Be careful in the tub with Epsom salts... things seem to get very slippery with them!