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The following plot is data I generated using 80 scouts to attack cataphracts with heroes such that no phracts die and all scouts die. I started with attacker having high honor and defender having low honor and kept attacking until the honor was reversed, and kept track of the respective honors and the honor drops. I've read elsewhere that the honor changes in the 30% heal range from 100 to 33.3% but it actually changes from 100 to 25%. Based on this data it is best to drop honor at the 30% heal range just over the 20% boundary (good heal and good honor drop) and worst to attack in the 30% heal range just before the 40% boundary (bad heal and bad honor drop). So look to drop honor on someone where (their_honor / your_honor) ~ 0.51 (just past the 20%/30% heal boundary), for example. In practice, this situation is probably unlikely so your next best choice is to drop honor when you are in the 20% range. You actually break even with the 20% heal when (their honor / your honor) = 0.5714, so 0.5 < (their honor / your honor) < 0.5714 is a good range, if you are lucky enough to find it. When hitting trebs, since the defenders heal doesn't apply, and you want the attacker to have good heal, it is best to hit in the 10% heal (50% from the attackers perspective) range.
For more details about the best regions to drop honor check this post.

The above curve can be summarized with the equations below, and a plot of the equation that describes the 30% range can be viewed here.
Code:
if th=0 and yh=0 then %honor_drop = 50 special case
if (th / yh) = 0 then %honor_drop = 25 special case
if 0 < (th / yh) < 0.5 then %honor_drop = 100
if 0.5 < (th / yh) < 2 then %honor_drop = 50 / (th / yh)
if (th / yh) > 2 then %honor_drop = 25
I'm not certain what the heal rate is if (their honor / your honor) equals exactly 0.5 or 2, but that is pretty unlikely to occur.
I want to credit Naems' Honor Dropping thread for documenting much of how this stuff works.
Raw data is attached.
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