I ran into same issue with having a 58.5" tub measurement from wall-to-wall. After extensive Google search, this professional (Terry Love) has this to say about the apparent non-issue:
"It's a 60" tub, but don't feel bad, you're not the first to jump to that conclusion. Tubs are 60" long, so most bathrooms are framed at 60-1/4" so that the tub will fit in. Then you add 1/2" backer board on both sides, that adds one more inch, then tile over that, which depending on the tile may get you to 58" finished. Its very common for a standard 60" tub to measure something like 58" to 58.5" from end to end with tile. The backer board and tile are built out from the walls so that water drops down into the tub and not behind the tub. Standard tubs are 60" Short tubs are 54" Long tubs are 66" or even longer." So, my conclusion from Terry Love is that a 60" standard tub will fit into a 58 to 58.5 wall to wall space size. You're welcome. lol https://terrylove.com/forums/index.php?threads/58-bathtub-nope-its-going-to-be-60.38234/
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