When I give partial credit for an answer, if the question is not graded on a numerical scale, I use the tilde symbol ~ because it means "approximately" in mathematical notation. I tell students about this before I return the first set of papers where I use this, though, because it's not a standard notation and they probably never saw it before.

If a quiz has five questions, and a student gets three √, one X, and one ~, that student would get 3 1/2 points out of 5.