|I have had a blind hemming foot for my serger (Juki MO-634DE) for years and finally tried it out today on 7 shirts that needed shortening. (4 men's undershirt tees and 3 RTW pique knit polo shirts). I discovered that when I guided the fold of the fabric next to the guide on the foot, per the instructions, I sometimes did not catch the fold. However, when I ran the edge of the fold directly UNDER the guide the fold was always caught and there were no unstitched gaps. The difference is probably 1/16th of an inch and worked beautifully. I also found having the R needle 1 # below the usual and the upper looper 1 # over the usual produced a nearly invisible stitch on the R side and was very flat. I used 3 threads as I don't have 2 thread capability.)
Another hint - the side seams on the RTW shirts were quite bulky to serge over, but steaming and pounding them first helped a lot.