I make a salt crusted roast every Christmas Eve. My family loves it. The only advise I could add after everyone else's great words of wisdom would be....
I double the amount of salt used. I never seem to have quite enough salt using only 1 box. I want to make sure I get a good crust.
Insert the thermometer and move it around a bit to move the salt around. You want to make sure there is no salt touching the thermometer or you will get high readings.
No matter how big or soft your bed is, you still have to get out of it.
-Grace Slick, raucous rock diva