I am no expert but common sense tells me this much:

You would need to count in late summer when the deer are coming to the fields. More deer will show themseves during daylight hours this way and the bucks are developing antlers so it should be easy to tell the sex. Next, disregard any fawn or yearling (just don't count them at all) because there is no way to tell what sex they are. Just count what you are sure of! Then you will have to do night surverys in the same manner. Again, I would think late summer would be the time for best results.