I used a stop block clamped to my miter saw's fence, so that all verticals will be exactly the same length. I also marked the board with a red stripe along one edge before cutting. The spar build-up is accomplished in several steps. This way, you can maintain more control over each joint. You'll have more clamps applied to each surface, and you'll be able to monitor clamping to ensure that there are no gaps between verticals, plywood, and caps.
Any gaps are really bad news, and are cause to start over.
This was my second set. On my first set I tried to do it all at once. The day after glueing, I was shocked at the number of glue joints that contained air, rather than epoxy. I vowed to do better on the next one. The first step of spar assembly was to glue all of the verticals to one of the spar caps.