As a practical matter the formula for a 100% jib may be recognized as the old tirangle area formula of 1/2 the base x height. Since the mast height does not vary...the measurement of the foot of the sail in relation to the 100% measurement will yield the actual % just as easily.
Example...I have a boat with a 50 ft. mast and 10 foot j measurement which yields a 250sqft. sail area.
Adding 50% to the J would create a 150% genny and this can be seen by multiplying 15x50/2 which yields 375 sq. ft. or 150% of 250sq. ft.
So...both methods are correct.
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