Measured one cm from the wall on each side, then used a chalk line between these. This line, which runs along the longest wall, I was planning to follow. I will probably chalk a few lines during the installation to ensure it stays straight. But by following the line, I think it's fine if the boards don't fit 100% since I’m following the line.
On the room's short sides there's a one cm difference, but I believe it's negligible as it won't be visible with a one cm gap along a 6-meter wall.