1 tablespoon per 5 gallons is what I did the one time I used salt and heat.
It was after I got a new fish and put it in the quarantine tank with my bala shark and tiger barb who were almost recovered from fin problems. I was not thinking straight and all three fish ended up getting ick.
My new fish didn't live long at all after that. Desperate to save the shark and the barb, I put about 1 tablespoon per 5 gallons into my 10 gallon quarantine tank and raised the water temperature to between 86 and 88 degrees. After a few days, all of the spots were gone.
At that point, I stupidly decided to play with the heater. I turned it down quite a bit for a little while (I think it was because the water was at 90 degrees...) and then left it there, forgetting to check on it for the rest of the day. The next morning, I woke up and checked the qurantine tank. The barb was doing fine, but the shark just kind of floating. Looking closer, I noticed that it was covered in more ick spots than you could imagine. Checking the water temperature, I found that it had fallen below 85 degrees. I turned it back up, but it was too late. The shark died later that day.
So, that led me to a new conclusion: It's not the salt but the heat that kills the ick. Even 2 tablespoons of salt did not save my shark when the water temperature was too low. Since that day, I've treated my fish with 88 degree water for two weeks. Haven't lost a fish since (knock on wood...)
I think that level of salt is okay for your fish. If anything, I'd raise the water temperature just a few degrees more. Due to my experience with my bala, I've become a believer that ick cannot reproduce above 85 degrees.
There's my two cents, hope it's worth it for you.