You need to be able to measure wind speed, barometric pressure, temperature, and humidity. Actually, humidity has a limited effect so it's not so critical. The Kestrel 4500 measures all the variables in one unit. It's expensive, but convenient. Less expensive models have fewer features, but you may have the ability to make some measurements already. If you own a handheld gps, it might have a barometer, or maybe you own an altimeter watch. Temperature is easy, and as stated before humidity is not critical.

If you don't have a barometer of some sort already, then the 4500 is the answer.

Barometric Pressure is measured in either inches or millimeters of mercury, abbreviated "inHg" or "mmHg".

The click sign (positive or negative) should be intuitive. Don't let the sign confuse you. If you have a 100 yard zero and are getting dope for 400y, you know you have to turn the dial in the "up" direction.