I was also a victim of focus shift. For years some were sharp and others not so sharp. Having a high end, big name lens did not eliminate focus shift. I found a top end 80mm Rodagon had shift and a 50 year old 75mm Wollensak showed none. I don't understand how that might work with Ed's answer, but it just is. When I started using a good Peak grain magnifier I had enough range and light to see the focus shift.
Yes, about two stops down is the sharpest for all my lenses. f/8 and be there.