I am istalling an americast tub. I need to shim it because if I do not the backerboard bows out at the bottom when it goes over the lip. How do I get the backer board level

I am istalling an americat tub. I need to shim so backerboard goes over lip level. How do I do this and what material do I use and how thick should shim be?

Naturally the shim should be the thickness of the lip to make the backer even to the tub's inner face of the lip.

Material:simple wood shim tacked in with finish nail.

Think of this,the shim won't be exposed to any moisture so using wood is more than acceptable,the spacing for the backer board needs to be even from top to bottom,or as close to even as possible,tapering the shimming can be done as long as the taper is enough to not be noticeable once the tile is installed.

Hope this helps.