Greetings @gimp,
Assuming you have a half decent chronograph I suggest you use your measured velocity. If you want to measure your down range velocity you may want to consider joining your local NRA club so you can use the Silver mountain kit. This will either correct or validate the predicted velocity. If the downrange velocity checks out then any difference in trajectory in likely to be operator error (sorry). If you can't measure down range velocity and wanted to change your predicted trajectory then I would adjust the BC and not the velocity. In spite of what some believe BC varies from shot to shot and day to day. I would also do some test firing at 300 metres to see how trajectory matches prediction. Last as mentioned above a decent aiming mark on you 600 metre target would help.
Regards Grandpamac.
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