Oh Boy! Another chronograph about to bite the dust!
I almost hate to comment on this, having destroyed a few of these gadgets over the years while using Muzzleloaders.
However, I can speak with some authority when I say 20 / 25' minimum, and that's if you're using sky-screens and placing cellophane over the sensors, 30 / 35' if you're using open sensors.
I think my last one (Chrony / Master Beta) recommended something like 12 to 15' using smokeless, and didn't recommend using it with back powder at all....same with no.7 shot.
It may be a good idea to call the manufacturer, or Brownell's, and see what they have to say. I know Pact, and Competitive Edge Dynamics (CED) will say not to do it at all, but the newer Chronys may be different.
Uncle Russ...