From the Pope & Young website..............

D. Let-Off for Compound Bows:

1. Definition of let-off: That characteristic of a bow that results in a reduction of the force necessary to increase the draw length after the highest level of draw force has been reached. This is characteristic generally associated with, but not restricted to, compound bows.
2. The maximum let-off on a compound bow shall be measured at a point in the draw cycle after the peak draw weight has been attained. It shall be measured near the end of the draw cycle where the minimum holding force is reached. This point in the draw cycle on a compound bow is known as "the bottom of the valley."
3. Determination of the percent of let-off: The values of the peak draw force and the let-off force shall be used to calculate the percent of let-off. The peak force is the maximum force obtained during the draw cycle. The let-off force is the lowest force reached following the peak force during a single uninterrupted draw cycle. In all cases, both the highest and lowest force shall be read from a scale during a single and continual pull condition, without relaxation. This technique eliminates the introduction of hysteresis, which can distort the reading.
% Let-off = 100 X [(Peak Draw Force) - (Minimum Holding Force)] / (Peak Draw Force)
4. Effective January 1, 2004, animals taken with bows have nominal percent of let-off greater than 65 percent shall be listed with an asterisk (*) in the Records. It is recognized that variations in draw length and/or draw weight can affect the percent of let-off on compound bows. For these reasons minor variations in let-off are acceptable.

So I think the 65% rule was due to downrange velocity issue's but with today's bows screaming ~300 FPS @ 80% letoff both the state and P&Y now recognize the 80% as ethical.
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