Knothead, I'm doing another test in my test line. Definitely my crossover looks much better now. I've stitched it and I'm at the point where I've milked everything back prior to marking for tapering. But a question (of course!): at this point in the fid's worth between where a core comes out and a cover comes out, it's thicker than normal diameter and tight, and no amount of pulling will make any more core go in, yet I've got maybe 3 fid's worth of core out at this point. Is that normal? If this is a prob, could it be because I'm working with a pretty short piece and didn't have anywhere near 5 fid's from the last mark to the slip knot?
Yes, that sounds perfectly normal. Once you have cut and tapered, the diameter in that area will be smaller. That's what the tapering accomplishes.
You do end up cutting off and throwing away quite a bit of the core. Don't let that throw you off.
And yes, it helps to have plenty of distance from the ends to the stopper knot. In the video I think I said 6 to 8 feet. You should probably have at least 8 feet maybe a little more.
Keep in mind that some rope has a tighter cover than others. Sometimes the same type will vary spool to spool. You may have just picked one that is more difficult.
Hang in there.