Each peice of land may be different, but basic information on herd health is just that...basic. There is no "different". You either have the right data or you don't. What your saying would only be true if there were some other factor, such as disease, that limits deer populations regardless of anything else. And you would only be able to tell that by looking at your basic data that you should have been taking in the first place. Trust me I do this for a living. But if it works for you then it must be the only good way.

Your not taking data really. Your making "observations" which are of somewhat less value from a management standpoint. To note trends and get good data you must make your observations in a repeatable manner. In the same place, at the same time, same conditions etc. Transect surveys are one way to do it. This will help you minimize the randomness of seeing animals. This will help you see reliable trends in populations, to a certain degree.

The suggestions about animal weight etc are good ideas. If I were you, I would be scutinizing every animal that is taken and doing a good B.C.I. (body condition index) and keep records of them all. This can really tell you what is going on with the herd if you do it for a few years. If you can see with confidence that your havested deer are in better shape that three years ago, you can probably assume that conditions/things that your not keeping track of are improving also. You can find the protocol for a good BCI on the internet I'm sure.

Another good thing to do would be to set up a few vegetation plots. Pick a few 10' x 10' squares and mark them off with string and stakes. Pick plots that are representative of the majority of the rest of your property. By seeing the changes in those plots over that years, you can make some assumptions about what is occuring on the rest of your property (that is barring any large disturbances like fire etc). Also, identifiying the plants within those plots and assessing thier forage value would also help you.