Help setting valves on a 225

Guys, I can't get a straight story on setting the rocker arm/valve clearance on my 1967/225 and I think that's what is causing some grief.
anyway, I was told to spin the crank until the opposing valve starts to depress, then use a feeler gauge and set clearance on the opposite one. Same with all the rest, so is this right? Or do I have to make sure the cam has the valve depressed ALL the way down???

Or is it best to do while it's running?

Confusing as hell.

Any help here?