Is it valid???/ yes and no. The no part might be when you set them out. For example, when the does are carrying fawns and have just had them they'll hit salt heavy and daily. By mid summer they'll just leave it alone except for an occasional walk by the old salt source. I'd say if you put them near each other at the same time you'd find within a couple days which they seem to prefer. But, all in all I think your findings are pretty cool.