Even with perfect measurements it’s impossible to get to that line straight unless the frame is perfectly square. This not only completely solves that issue but also saves a ton of time not having to measure and cut them one by one.
Also the lumber I picked turned out to be 10 cm longer than the deck and I even picked the exact right amount. By the time I was done I only had a basket of <10 cm off-cuts.
I stand by what I was taught by my grandfather and my father.
Only you’ll care if it is off, but if someone comes to the house and says it is wrong, hand them a saw and say get to work.