I rarely follow this part of the site but saw this question and wanted to add my 2 cents. Jim is correct about putting space between the bottom of the arrow and the top of your index finger.
I shoot what I called a modified string walk/gap system where I have a knock point above and below the arrow (arrow should little or no movement up and down between the arrow nocks for good arrow flight ) and and an extra knock 1/2 to 5/8 of an inch below the bottom arrow knock. I don't walk up and down the string like a string walker but people at first glance think so because where i grab the string 3 under is not touching the bottom of the arrow, it's 1/2 to 5/8 below the bottom nock on the arrow. I have nothing against string walking but i set this system up for broadheads and only wanted 1 drawing point to avoid eradic arrow flight. I started using this idea in 2007 when i had the same question and missed numerous deer because of the 1 1/2 foot gap i had at 15 yards. Now, i can adjust that bottom nock point so that im shooting point on to where i want....25 yards and my gaps are about 4 inches below at 10 yards, 3 inches below at 15, point just under target at 20, arrow on or covering point of aim at 25. This system is not for everyone and I have had some people complain its not traditional.... And I can't shoot competitively in some tournaments because some people call my system string walking even though I pull on the string at the same point every time.... Or that I'm not touching the bottom of the arrow.... Or that I'm not shooting split finger. With respect to 3d shoots, I only shoot them for fun, to be with good friends and my 9 yr daughter who truly shoots instinctive (and shoots instinctive well, I'm jealous) and to practice for deer season. I have heard it all as to why i cant shoot like this "for score" (usually after i already said im just shooting for fun and no score) and choose to not argue with anyone about whether my system is legal for competing or not .
The system has helped me be more effective hunting because its easier to judge a 3 vs 5 inch gap vs 20 inch vs. 15 inch gap...good luck. I hope I made sense. If I didn't and you have more questions or want a photo, pm me. I just learned how to iPad and email but I don't know how to post photos here. Bhill