I'm sure this subject has been discussed many times in the past but as an angler of over 35 years I still have bother with the dam things

I'm not up to speed with the names of to many knots but the main dropper know I use is the one where I get a 10" piece of cast for my dropper and measure it back 3ft from the point fly, I lay it onto my main cast and wet them both, I make a nice loop of about 2.5" and bring both the dropper and main cast through 3 times and as I pull tight I wet them both with a good old slobber, then come my delima ~ which tail part do I trim off ? I just normally trim off the piece that's the shortest leaving the longest part as my dropper, the knot it's self never lets me down strength wise But the dam thing twists and turns around my maid length of cast after a half dozen or so casts, why why why does it do this ?
nige