"The first part of the answer in determining how deadrise affects
performance is another question: deadrise measured where? If the angle
between a horizontal plane tangent to the keel and the chine is
measured just aft of the bow, most modern boats will measure around
45° or more. If measured just forward of the stern, the deadrise may
be as little as 5°. The days when deep-V hulls carried the same high
angle of deadrise nearly constantly from bow to stern is long gone.
Why? Because they didn't perform very well."

Hey John, this is a pretty fair explanation of the reason for using a
measurement somewhat forward of the transom for a published spec...It's
not so much of an average angle, as it is the angle at the widest
"wetted" area of the hull at speed..the low deadrise at stern provides
stability, especially with engines mounted well aft.. a high angle
carried all the way aft tends to make a boat "porpoise" at speed...too
low an angle causes instability when turning at speet, too rapid a
transition and you get a boat that meanders at low speed, and can
develop low pressure areas at speed that can negate lift or planing
ability..