Newbie tiler: planning the first row

I'm attempting my first ever tiling job -- a part-wall tiled splashback behind the sink taps and around the kitchen window. I am using 15 cm tiles.
The tiling area disappears behind the fridge. So I am attempting to put full-size tiles around the window if possible and then hide any cut pieces behind the fridge!
Here are some questions:
* I want to put a row of full size tiles under the window ledge. The window ledge seems to slope. I fixed a plumb batten at 15 cm under the ledge. I can fit full tiles at the LH side but as I move right the gap is more like 16-17cm. Looks like I am going to have to move the batten up slightly, trim 1 cm or so off the LH tiles and use full tiles on the RH side. Or does the batten have to be exactly plumb? Can I just bang full size tiles on and carry on regardless?
* I intend to use a plastic joining strip where the tile rows meet at the edge of the window ledge. Should this strip be inserted under the row on the window ledge or behind the row under the window?
* The window ledge is uneven. I now know why I found a layer of part tiles beneath when I took off the old tiles -- it was to attempt to build up a level layer. Any tips?
Thanks Bruce
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Can't you build the window ledge up to be flat?
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The
the
gap
batten
the
bang
Doh! Its obvious now! I just have to put a few patches of polyfilla and scrap tiles on the LH side of the window ledge. Should even it all up a bit.
Thanks. Bruce
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