ok, it's not plaster, it's actually render but the application is the same(ish).

I'm rendering the inside of a fireplace, trying to get the two joining walls and the floor to be as near a square as possible such that the subsequent tiles all meet correctly. Is there a technique for getting the back and side walls 'vertical' to each other. If they are NOT perpendicular then the tiles will show an increasingly large 'taper' in the grout that goes into the corner joint unless the tiles are cut to account for the taper - then the tiles will look 'uneven'...... won't they?

Is there a maximum amount of 'outness' the verticals can be before it becomes noticable?

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Square up the area by screeding.

The tiler can work with a bit of tolerance in a small area, Don't focus just on level, try and make sure all the surfaces are square because this is more important. If you are in any doubt, design the tile layout and then render to make it easy imagining you are the tiler.

Put a screed (strip of render) up the wall in the corner,

check it for plumb with a level, and put/take more on/off as necessary to achieve a flat and plumb screed,

do the same the with the other end of that section of wall,

when it's stiffened up a bit fill the gap in between the two screeds and rule off to them with a straight-edge.

Repeat for the other walls of the recess.

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One day it will all be firewood.

Now 'that' ^ seems ike a simple and workable solution - thanks

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You will need a square when screeding.....