If you're dry dashing don't let it pull in too much. Make the receiver almost fluffy, so it spreads, if the stones are bigger, obviously need a heavier coat, but generally you're not too bad depth wise, slightly less than a float coat,but heavier than a scratch. (way we do it anyways) coat it on nice and flat, coat up and down then trowel across it, and flick the stones at it. If you let it pull in, the stones won't stick right, will bounce off. If it's roughcast, better to let it pull in a bit, but it sounds like normal dry dash uv described