Setting Out Setting out advice please!

[macten]

Sure it won't be!
One thing I would recommend you do is when you remove the 20mm board off the cill, tile the window wall before you add the thinner board. You want it bang on level with the tops of the full tiles so take any guess work out the equation and build up to the required height after tiling.

 

[LEH]

Yeah I was thinking it would be a major pain if it were a mm or two too low, given how close it is - will certainly do as you suggest!

 

[Albert]

Yeah I was thinking it would be a major pain if it were a mm or two too low, given how close it is - will certainly do as you suggest!

It would be a bigger pain if it was high