On Sun, 18 Mar 2018 14:32:50 +0000
John Rumm wrote:

(although there is an argument that if levelling something close to
another horizontal surface, it often looks better to copy any error
in it rather than fix it)

I fitted a mantle shelf and tiled down from there, only to find that the
tiles on one side didn't quite meet the hearth because it was "level"
with the floor. :-( Just painting the wonky grout gap made it
blend in sufficiently that the unevenness isn't noticeable, but that's
not a mistake I'll make again.