I've got some info somewhere on the WGr Jan, but far as I remember, it was approximately 1,000 metres stand off maximum, for reasonable acccuracy, with proximity fusing, possibly accoustic proximity fusing also, tuned to the Db output of the B17/B24 engines.
So, basically point and shoot, and if detonation was close enough, it would cause serious damage, or knock the tatget down. The round was basically the same as the 'Moaning Minnie' ground 'nebelwerfer' ammunition, so a realtively wide blast area on detonation.