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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.
This is under a roof so it doesn't get wet regularly. However even if it did that would be okay due to the NTR-AB grade treatment on the wood. It resists water for as long as it gets to dry out in between. For framing I use NTR-A grade which can withstand being constantly wet but there the cut ends needs to be treated or else that's where the water gets in and it'll rot from the inside out since pressure treatment is only on the surface.