You can put the betadyne right on the snake straight. Or draw a bath at 88.f and pour beta in until it's tea
Colored.
Or you can mix a 1.1 ratio of beta and water.
Betadyne is incredibly safe it won't harm the animal to wash him in betadyne.
When I've dealt with scale rot I'd do the soak, dry the snake off well and then apply beta directly before going back into the tank.
Make sure you have a hospital sterile enclosure with no dampness.