The best way I found was the head dunks. Rubbing the salt into my scalp was okay as well, but not as effective. For the soaks I was using about a teaspoon and half for about 2-3 litres of water. It was never precise thought, I just kind of put in as much I felt would make the water similar salinity to ocean water.
Not sure how this would work out, but looking back I might have considered adding a drop of iodine to the water if I was still going with this treatment approach. Way back I tried directly applying pure iodine on the spots, but that was way too crazy and slightly burnt my skin.
It’s strange to see why the salt stops working so quickly. For me it was quite magical at first too, for about a week I was riding high and thought I had everything figured out. Then it kind of just stopped working.
Even now though, I still think sodium and iodine might play a role however. I eat quite a bit of iodine rich foods and I’m not really watching my sodium intake as intensely as I did before. This seems to help the sweat glands behave more naturally.
Let me know which route you end up perusing. Best of luck either way!