Terrific job! It is refreshing to get data, and your home tests are valid comparison data. The only suggestion I would offer for statistical significance would be to do each test a greater number of times.
Re usiing a needle for a nail knor or a uni-knot, a Geoff Wilson book illustrates that the unit-knot and nail knot are identical, tied by different methods. I proved this to myself by tying dozens of both with small diameter cord.
I wonder which change, the double overhand or the nail knot, conributed the most improvement?
Many thanks for this work, Juro.