A real lot? You buy a mitre saw
You screw it to a bench, then you cut a block of wood and screw it to the bench exactly 600mm away from the blade, then you use it as a stop to position your CLS, cut, remove, repeat
If by circ saw you mean one of these
You can make a jig with some timbers screwed to a bench and form a thing where you can slide a timber in, then run the saw along a fence and it will cut exactly 600
Blacks the cut line of the saw, solid orange is the fence and the stop. The support timbers need to be far enough apart that they don’t get cut by the saw . If it sticks off the end of the bench, then all the cut bit will drop out.
So it’s not really the best for this job unless you’re cutting a lot of timber and you want to lay out multiple studs next to each other and cut 600 off all of them in one pass
Also note if you need exactly 600 you’re probably gonna have a fair bit of wasted on every timber because there are multiple of 600 long and you’re so late does have it with so by the time you’ve run four or five cuts you’ll end up with a piece that’s eg 596 long