I am remodeling a bathroom and I am putting in a new tub and then tile the walls of the tub. My problem (right now) is that my rough opening is 60-3/4" at the back and 60-1/2" at the front. This leaves me with a 3/4" gap at back and 1/2" at front. My backer board has to butt up to sheetrock on an exsistng wall. How do I fill in the gap, which will be almost an inch, behind my backer board after I set it inside the lip of the tub but still keep it where my backer board will butt up to the sheetrock and be on the same level.
I don't know exactly where the other wall is you want to match up with. The standard method is to shim out your rough opening to be exactly what you need for the tub. That means putting shims on the studs around the tub to get it lined up where you need it.
Once the tub is in where you need it, add more shims to the studs above the tub to bring your backer board even with the tub. If the lip of the tub is 1/4" you will need to add at least 1/4" shims the entire length of each stud (maybe more if you have more space on some studs). I used wood lathe for my shims, if you have a table saw you can easily make these shims yourself to whatever dimension you need for your tub.