M_Boats: Intermediate jib size

Spring sailing season is approaching on North Carolina's Neuse River. Winds in the high teens and 20's are common. Prior posts have stated that an "intermediate" 60-65% jib of between 20-23 square feet was really useful on San Francisco's Bay in similar conditions. Does anyone have the luff, leach, and foot dimensions for such a jib? Jerry M, I believe, once said that a jib that came to about 18 inches of the hounds was a good sized intermediate. Does that mean that a full sized M-15 jib could be cut down by removing 18 inches across the foot? Seems like you may want to cut less at the leach than at the tack.