The ledger board is how many of us install a cast iron tub.
If the home is not new, put a level on the floor 30" from the back wall to see which part of the floor is high.
The apron will touch down on the high side, and the low side will need to be shimmed.
The height of the ledger board is determined somewhat by the high spot you measured.
The tub must be installed level front to back, or tile will look awful.