Drifters, are much longer in the foot than most headsails. You can give us the I and J measurements, (refer to the basic genoa measurements), much like the headsail measurements. You can measure the luff of an existing sail to give us an accurate luff length. If you presently have a genoa that is 150%, the foot length is 150% of the J measurement.
A drifter can be longer in the foot if your boat is set up for the increase in foot length. We generally give as much lenght as a cruising spinnaker to allow for more sail area. If you find that your genoa cars are not back far enough, then you can still produce a light air genoa to enhance peformance on windless days and still point higher than a cruising spinnaker. Refer to the instructions for measuring for a new genoa or jib.