I agree with DBin, I switched to 3 under because I anchor a bit low, my thumb knuckle to the rear of my jaw bone, so it got the arrow up a little closer to my eye, although it still is further below those who anchor their index finger to the corner of their mouth or eye tooth.
I shoot completly instictive, I pick a tiny spot on the target and consentrate on it, that being said, I also let the tip of the arrow into my site picture but in no way use it as a reference point in conjuction to the spot I am trying to hit. I just think by letting the arrow tip in that my subconsious mind will program itself in telling me where to hold depending on my distance, I also do not pay any attention to what yardage I am at, I let my internal computer take care of all of that.
I have heard more then once that shooting 3 under is more forgiving as there is less chance of arrow torque on the nock when using split finger, don't take my word for that, just something I have heard alot of shooters say. DK.