As has been suggested before, get a digital 10 to 12" magnetic base digital torpedo level and the Prop pitch measurement jig tool from Van's to get it fairly accurate.
Adjusting your pitch once or twice, fine tuning it, will make the process seem more simple and routine.
Find a way to lift the tail under the eyelet with your wheels chocked to get a dead level measurement on your longeron with your digital torpedo level first.
The rest of the procedure is pretty easy.
I like to put a piece of tape with a very fine inked line where the split in the front and back prop hub meet, for a reference mark, before you loosen up the bolts and start adjusting.
I also write down and measure the pitch indicated by the gauge on both props, BEFORE I loosen the prop hub bolts up, for a reference.
For reference, only:
Another RV-12 user with a 2011 observes the following performance flying 1000 ft over the ocean with temps around 72F. He has his pitch setting for cruising at 8500 to 12500 ft, he's 90 # under max gross with wife in the plane.
At 90 kts climb speed, 5200 rpm
at 85 kts climb speed, 5150 rpm
at 75 kts climb speed, 5090 rpm
N836BL likes and prefers a fast climbing plane. His pitch is set at what Van's recommends, and with both of us being right at 1320 with 12-14 gals in the tank, he sees the following.
At 75 kts, 5150 rpm to 5170 or 80 at 1000 ft ASL on takeoff. Usually 60 to 75 degrees F and density altitude at 1300 to 1900, sometimes as high as 2700 on a hot humid low pressure day, with onshore fog that's burned off finally. We almost always see a minimum of 800 fpm climb rate, usually 900 fpm. He likes his plane to climb fast. At 5200 rpm and 5000 ft, it will easily cruise at 112- 115kts TAS, no problem.
My plane is still pitched too course, at 75 kts and 5020 rpm. I'd like to see at least 5150 to 5200 rpm at 75 kts climb and hopefully 900 fpm or a little better, near max 1320. My plane will pull 5600 to 5650 full throttle in level flight at 3000 ft elevation. Right at 120 kts TAS. ;-)
Good luck and let us know how it turns out.