Good sir, are you SUGGESTING that I might have mis-measured?!?

Er, wait... hmm...

Mystery solved. I initially cut all of my spacers to 10" before noting that the 10" referred to the spacer PLUS form. Turns out I neglected to trim a couple down to 9.5"! Worse, those ended up being placed at the center of the strong back- when double, triple, quadruple checking my spacers I'd get about five in from the bow or stern before convincing myself that the error must be elsewhere!

I'd like to say I'm relieved that I'm not losing my marbles but the resolution wasn't much consolation!

Thanks for humoring my error, all.

Patrick