Seen guys do that but most barometer's are not accurate enough.
In my opinion the aircraft alt is the way to go.
Far as temp and humidity...there is a radio shack digital deal that is not expensive and seems to work well.
Someplace that repairs airplanes. Small airports. I paid $100 for mine years ago.
Actually I use a special calculator that is very old.....LOL...and if I remove the batteries to replace them I will lose the program. But I found something on line that is accurate to my calculator....whenever it dies I will use this.
I just saved the web page to my laptop and the calc works even when not on line.
I suggest you set your aircraft altimeter to 29.92 and and also enter 29.92 in the calculator.
A lot of times when racing....we look for the "direction" the air is going. If I see the altimeter changing.....I may change the tune up anticipating the air coming in or the air going away. An Aircraft altimeter is very sensitive while many others are not.
Adjustments to the motor depend on your experience and the air. While a change from 1500 ft DA to 1800 ft may not be a big deal.....changes above 3000 ft ( changing from 3000 to 3300 ft) may be a big deal.
Humidity plays the least factor. Elevation and temperature play the most.
the other person said it correctly" you have to use one consistantly" so what are you going to do if your friend is there or he's in the water and you don't know how to use it, answer, get your own. i've got 1 in a nice carrying case for sale if you're interested. send me a pm