I have been using distant street lights (at night) and a sinlge wall light in my living room. I'm now fairly certain that I have it in the right place at the 70.5mm distance specified in the Schneider data on line. This would make sense as I wouldn't expect their data to be wrong.
Wide open is only f8 on this lens which gives quite a bit of DOF which also equates to margin for error.
I think a film or two will tell me what I need to know.