I've read many posts from people who have such a hard time making a Y seam. Most of us, including me can not make the joins come out where they need to be without pinning. I took a tip from a clothing manufacturer where I used to work at and made "drill" holes. I'm not talking about something huge, so please don't gasp in fright!
When I start a tumbling block or something similar, I stack the pieces, 20 or so high. I mark the top one on all of the intersecting corners. Using the biggest machine needle I have, I walk it unthreaded into that intersection or matching point, wiggle it just a bit, then roll the needle out. Now, I have my diamonds ready to align up. I can pin if I choose, or at least have stopping points to eye-ball it.
PM me if you need more information. Seriously, if you can't get it right using another method, you are going to have several holes to deal with anyhow.