Have you bought the tile yet? it can be anywhere from about 3/8" to nearly 3/4" thick or even more. The cbu is nominally 1/2" (note Hardibacker, although called 1/2" isn't - sort of like plywood these days), but the exact dimmension depends on the brand. If the tile is actually a slate lookalike tile, then the thickness is probably more consistent. If it is real stone, then you may need mediumset mortar vs thinset, so the actual thickness is subject to the consistency of the tile.
Have I confused you yet?
If tile, then add 1/2" + say 1/8" for the thinset + the thickness of the tile. The thinset may end up thicker or thinner, and the cbu may be like 7/16" vs 1/2".
You really need to measure the real things and then fudge the thickness of the mortar.
Don't forget to put a vapor barrier over the studs before you put up the cbu.