This is an actual situation that occurred recently.
We received an application from a handicapper stating that the boat he had at his club was a Modified boat.
Reason: The Measured "P" dimension is not the same as the Standard Boat that we have rated. It is 2 feet shorter.
After being measured by our in house expert, it was determined that this was NOT a Modified boat.
In house expert: I measured the I and ISP. If you ask him how he got his measurement, I bet he will tell you that he measured from the top of the mast to the cabin top where the mast goes through, that is 2 ft. shorter.
ISP and I are measured to the shear-line, which is where the hull and deck meet.
You put something straight across the life lines at the mast and measure down at the side of the boat to where the hull and deck meet. You measure both sides and average it.
You make a mark on the mast where the straight edge touches the mast and then measure from the top of the mast to this point.