Shower valve may be 1/8" too far back -- will we be OK?

Not sure where our calcs went wrong but we've got an American Standard R540 valve installed--very difficult to move at this point!--and the tile guy says that the tile will end up about 1/8" proud of the Max dimension on the plaster guard. I understand the meaning of Max... but is there likely still some flexibility here are are we going to be screwed if we install the tile w/o moving the valve out?

A deep rough kit can be ordered

It's been my experience that Max means just that, i.e., if the valve is recessed too far, then stuff doesn't work, like
you can't screw the handle on. There is plenty of leeway/flexibility in the range of depth given in the specs, and it is
almost always a problem when people try and jam the valve up to one extreme or another.

Was afraid someone would say that. Anyone have experience with this particular valve or similar that says differently?

Your choices generally are: move the valve, or search out for an extension kit (which may or may not be available). Most people don't like the look of the handles sticking out further, but that's what you get for not getting the valve-to-finished wall distances within specs.

At 1/8", I might try loosening the valve to the blocking and try to wedge it out a little, but that's generally not a great idea (but 1/8" isn't much!).

IT strictly depends on the trim for the valve. Some max/min dimensions are "suggestions" with a lot of latitude, while others really mean it when they say "maximum". At least, if you are too deep and it does not work, you can get an extension. It is when it is too shallow that problems occur, because you cannot get a "shrinker" to make things fit.