Bath gap to wall at first fix

Hiya me again !

I measured 170 exact between walls where the bath will go and its not worked out.

Its tight and not square and i dont have any wiggle room to seal it in (is a metal bette bath so not sure if i need to tbh on that but still it is tight and im worried about wall movement etc.

how much room do you sensibly need around the bath ?

Option 1

I think im going to need to redo this stud section and move it back breaking the tanking but that isnt a exposed wet end tbh and i can always redo this bit. I can do that back by about 10mm

Option 2

another option is cut the 10mm tilebacker below the eventual tile line?

Option 3

Replace with 6mm (ive got some hardiebacker but is 4mm enough?

I need the section to run along to the eventual bath cover as well to add to the issue.

Help appreciated

Because im an impatient cowboy I went with option 1 by 5mm to give me some wiggle room.

Only thing I didn't remember was I'd scrimmed before doing the tank up before in the corner but never mind.