"P" is the luff length of the mainsail, measured along the aft face of the mast from the top of the boom to the highest point that the mainsail can be hoisted.
"E" is the foot length of the mainsail, measured along the boom from the after face of the mast to the outermost point on the boom to which the main can be pulled.
"I" is measured along the front of mast from the highest halyard to the main deck. The main deck is where the deck would be if there were no deckhouse.
"J" is the base of the foretriangle measured along the deck from the headstay to the mast.