Did Loren say that??
I think he knows better-maybe a long day at the office?
The "E" measurement refers to the distance from the back face of the mast to the point on the boom where the clew of the mainsail is designed to be located at max tension-or put simply, the "E" approximates the foot length the mainsail, less deductions for the tack setback (or where the tack pin is)about 1" aft of the back face of the mast.
The "P", which approximates the LUFF length of the mainsail, is the vertical distance measured from a line (often marked with a band or tape) on the mast even with the top of the boom up to the point near the masthead where the head of the mainsail will be when fully hoisted. This will usually be a few inches below the main halyard sheave..In thinking of the mast and boom forming a right triangle with the leech of the main as the hypotenuse, the P and E are the base and height respectively and refer to the gross LUFF and FOOT lengths for the mainsail.
The "I" is measured from the top of the genoa halyard sheave down to the base of the upper shroud chainplates (well-that is the closest point you can actually measure with a tape-the actual location is called "base of I", and is slightly lower). The "I" dimension forms the height (long side) of the right triangle forward of the mast (the "foretriangle"), with the "J" being the base of same. The "J" is measured from the forward face of the mast to the stem fitting on the bow-the headstay is the hypotenuse of this right triangle..
None of the I,J, P,E are actually measured to the masthead.
IIRC, the P for the std. rig 27 is 28', and the E is 10'
Hope this helps,
S