I use MF medium a lot and to soften up my baits I might use 1 teaspoon to 4oz
Only an estimate of course as I don’t measure out that amount.
To make a bait extra soft start with 2 or 3 teaspoons and go from there.
Your idea of super soft may be different than mine .
You will more than likely have to experiment with the softener until you get what you want.
When I make stick baits I’ll use MF sinking soft add my salt or glass beads and then use softener there too. Typically about the same as what I use in medium, and even then I may have to adjust the amounts.
Start with plain plastic. Add a teaspoon of not what you want. Carefully reheat and add another teaspoon. Still not what you want?
Repeat as necessary. Might take a few tries but once you get your desired softness write your recipe down