Nice rifle, what are you sighted in at presently? 500 yds is a lot farther than most folks think, but your 308 will not have any trouble with that. Do some ballistic table research for your specific bullet weight and velocity and you should be able to make a darn good guess for your hold over. Likely not perfect but it will get you on the paper than you can dial it in from there. If I am walking out there cold with unknown customers rifle I will typically stack two targets vertically then aim at top to establish drop at misc distance provided I have verified the HZ is basically accurate. And remember the wind is not your friend! Pictures or it didn’t happen!