Need help with my quilting blocks measurement?

You seem determined to do this the hard way. If it were me I would have 3 inch borders (cut 3.5) and blocks finished at 15.5 by 14. cut the block 16 by 14.5. Measuring with the Omnigrid would be easy if you cut them this way. It would make the quilt 65 by 87 though. but hey, I want my quilting to be fun.