Howdy, MrSargyano.
The 3:15 min rule is to metric measure as the 3:00 min rule is to imperial measure.
Mariners use these rules to determine the speed of another vessel under their observation.
E.g. If it is determined that an observed vessel travels 0.8 km in 3:15 min., then,
the observed vessel's speed = 8 kn. If 1.5 km, then 15 kn. etc.
Imperial: Distance (yds) in 3:00 / 100 = speed in knots.
-...-
btw, nice Pickett
sweet.