What distance for chronograph?

I usually have it 3 feet from end of barrel. 

I usually shoot from 12-36” with Competition Electronics Pro Chrono.

Slightly beyond the vapor cloud emitted from the muzzle. For the Condor, usually 3 feet for my Oehler 35P chronograph. For my other less powerful guns, much closer.

Atmospheric conditions are a factor too. On humid days when the temperature/dew point spread is low, the vapor cloud will be thicker and extend farther, so move the chrono farther away. On dry days, you may see no vapor cloud; in which case you can move the chrono closer.

Experience will teach you what distance is appropriate for different guns and different conditions.

On my small cals I go 12" more or less. On my big bores, I go 5 feet. if you are testing projectiles against each other you should use the same distance each time. I usually put a level on my rifle barrel & make the chrony match that. Naturally consistency gives you more accurate data. 

Never measured it but about 18 to 24 inches, shooting Marauders 177 22 and a 25.

I get the best results for my .22 cals the closer I can get the chrono to the barrel end. I shoot it at 9 inches away from the barrel or less.