The weather was clear in Mn. this weekend, so I screwed the gun together and took it out to the range to decide where to locate the back sight. No shooting was done.
I set up my spotter at 60 yards. Note: I just joined the Wild Marsh Sporting Clays club recently. When I told about what I was trying to do, they went to the 100 yard range and put in ground pipes (to hold a target frame) at 60 yards for me. When I was at the range Saturday, one of the owners came by to check on me and make sure everything was satisfactory! I like these people!
I am using a pinhead Swiss front sight, so I black taped it on the front. I had marked the back of the barrel at 6,7,8,9,10,11, & 12" from the barrel end. I then covered the top of the barrel with double sided poster tape. I had a handfull of back sights with me to play with, but I am concentrating on "U" and square bottom notches around .040" wide.
I would put a sight at 8", study the sight picture, then move it to 9", then move it to 10"and so on. After a couple of hours of this, I was repetitively going between 9" and 10" and settled on 9-1/2".
Besides locating the back sight this practice showed me that I want to shorten the comb to move my thumb back about 3/8" or so and to push the comb (which hasn't been rounded yet) as low as my design allows.