I use an Oregon Scientific. Usual stuff, temp, pressure, humidity etc.

I recommend it!

You will of course need to get a good base setting to start with and then take lots of notes of the conditions. You'll then be able to get pretty much in the ball park next time you go out.

Remember there's lots of things that affect jetting so record as much as you can.

Temp up - Jet down (air less dense, less oxygen/volume of air)
Pressure up - Jet up (air more dense, more oxygen/volume of air)
Humidity up - Jet down (water in air displaces oxygen, but only small differences here)
Altitude up - jet down.

That's a very simple guide.