Hi waynewie, welcome. I usually set the tile on the deck first, get it nice and level and then set the tub in the hole.
The mud needs to be wet like wall mud (fat mud). Sometimes it's advised to lay a layer of plastic (poly) over the wet mud then set the tub on the plastic and mash it down. The plastic keeps the cement from sticking to the bottom of the tub in case it ever needs to be pulled out. It will still fit the shape of the tub and give it support. We don't ever measure the mud thickness, just flop down 5 or 6 shovels of mud and push the tub down into it. As long as the cement is touching the tub, it'll support it.
If you have a wood floor, staple down felt (or poly) over the wood and a sheet of wire lath so the mud will stay put.