There's nothing wrong with current or previous rules. J is a standard measurement and as such is a single number, like LOA. So 11.5' (or 11' 6") is accurate.
J/30 Class rules allow for 13mm or 1/2" for the tolerance in measurement (rule 4.1). I'd be surprised if the LOA is within +/- half an inch. Also, don't expect to get 11.5' measuring along the downward sloping deck from mast to stem as J is the horizontal distance between the two.
