Sorry not sure what you mean by a waterproof band. I am putting a 6mil vapor barrier behind.
Winger,
Use your level and carpenters square "first" to get your framing right.........Make sure corners are plumb and square and your walls in plane FIRST...You minipulate this with simple drywall shims and force it into your best version of plumb square and in plane .!...... Then assess with said level if you are clear of tub flange.
If your level does not slide down the wall and clear the top of flange and you want it to, use more drywall shims equally to fur the walls out and meet your clearance demands.
This will also help you more than you know when it comes time to layout and tile, and will result in the best outcome visually.
If you are using a vapor barrier behind cement board..... "do not waterproof topically"..... If you are dealing with an exterior wall in your alcove you will need to meet local code and vapor barrier/insulation specs. This local code creates issues with waterproofing and you should refer to tech support from the specific MFG in question to resolve before moving fwd with a vapor and waterproofing combo.
If A guy named Jim stops by with a pair of red converse shoes on RUN ! as he will confuse you even more.
That is how I would approach your situation if you had hired me PROFESSIONALLY to prepare your walls for tile. -Redshoe