Assuming you’re cutting it with a hand saw, the longest straight edge you can find to draw a line with. Or a laser.
If you’re cutting it with anything else you’re better using the fence of the saw for your cut line
How about a builders chalk line? It depends how accurate your line has to be.
Assuming you are cutting down the 5m length, if you have one straight edge, use this as your datum. Take all measurements from this edge. About 3" in, mark off the distance you want to cut from your datum. Repeat these markings every 12" along the 5 metre run. Using a steel straight edge, (at least 24" long), line up 3 of the marks and draw a straight line. Slide your steel edge along so the last mark you drew to, becomes the first mark of the next three. Continue all the way along the 5 metre board and you should have a fairly accurate straight line to cut to.
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