Gaps created by cement-board being further that the bathtub-lip when tiling?
Either your way or Ed Beal's are fine &amp; do work. You just don't want any voids or unsupported portions of tile to avoid impact cracking or breaking. Back-buttering &amp; going to a wider trowel notch should do it. But otherwise, mark your layout if you fill-in, to make sure you're actually filling to the row's edge.