Measure once, cut long.
Test fit, measure again, trim shorter.
Repeat until fit is correct, then install.
That sounds a bit smartassy, but I spent all my spare time over about 10 weeks of this Summer single-handedly building a deck on the back of my house, and there’s nothing worse than cutting too short. I don’t build for a living, or even a hobby, and I found the inconsistencies of dimensional lumber make measuring and cutting correctly extremely important and sometimes frustrating. Intentionally cutting long to facilitate a test-fit-then-adjust workflow became an extremely valuable approach, especially while building the railing.
