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.
What tool to cut them off without damaging surroundings? Just a circular saw?
Circular saw with the blade set such that it just barely will cut all the way through.
Shouldn’t you set it about 1/4” below whatever you’re cutting?
Depends but i would say quarter is way too much. Generally dimensioned lumber is close enough that right on is just fine but rough cut needs more tolerance. My habit was often a 32 less and the rest breaks off with no danger to the blade.
Track saw or a circular saw