“How do add a putt that is a tap in? I’ve been adding it as a 1 foot putt, is this correct?”
Yes, input as 1 foot.
“For tap in putts, is it possible to record your putt in inches, or is 1 foot the smallest? ”
1 foot is the smallest. There is no difference in SG between 1 inch and 1 foot, so no need to distinguish.
“Even if I two putted all of those , surely that wouldn’t be a handicap of 30 worthy?”
From 7-21 feet, the average strokes to hole out is less than 2.0. For example, for a scratch golfer from 17 feet is about 1.9. To simplify, let’s ignore 3 putts. Then the one-putt probability is 10% and the 2-putt probability is 90%. So a 2-putt from 17 feet has an SG of -0.1. Do that on 18 holes and you’ll lose about 2 strokes putting in the round and your putting handicap would be about 10 (give or take).
Now going through the same analysis from 10 feet, i.e., 18 two-putts from 10 feet, each losing 0.3 strokes per hole, would have a total SG putting of around -5 to -6 and a corresponding putting handicap of around 20-25.
You’d get a handicap of 30 if you also had some 2-putts from under 7 feet.
If you putt from 10 feet and leave yourself a 5-footer, your SG on the 10-foot putt will be much worse than if you leave the 10-footer 1 foot away. The SG from 10-feet doesn’t care whether you make or miss the 5-footer, for that counts for the SG for the 5-foot attempt (and goes into the 0-6 foot SG bucket).
So it seems like you missed every attempt in the 7-21 foot range and some of the were not tap-ins. Your 0-6 SG was great, consistent with your comment about two-putting them all.
Not all two-putts are created equal – a 10-footer hit to 5 feet and a sunk 5-footer is not the same as a 10-footer hit to 2 feet and a sunk 2-footer.
Also, the handicaps for one round can be quite variable. See what happens with a few more rounds of data.