Problem with tiling

[greatgrey]

I am tiling my shower room from floor level to the ceiling. One wall has a small recessed window. My wife wants a 1.5" border strip incorporated in the tiling that will go directly below the bottom of the window ledge. If I work to the existing window ledge level the other wall tiles will finish 3/8" from the ceiling. The only way I can correct this is to build up the window ledge height by 3/8". Can anyone suggest the best way to do this please.
The window ledge is about 24" x 7" and at 3/8" poly filla is out of the question. If I purchased a small bag of ready mixed sand and cement would it adhere to the plaster finish on the window ledge?

Thank you for any advice you can give me.

Greatgrey.

 

[Richard C]

Much easier way than that; get a small sheet of 10mm plasterboard, cut a piece to the size of the ledge, key any paint by scoring it & stick the PB on using a trade grab adhesive & tile directly over that.

Might be worth some though about where you run that border though; personally I think they look better running over the ledge so they stop in the window reveal rather than running across & under the window.