I don't recall whether the 1/20 factor is correct from my own tests, but the scaling is correct.
In 1.2 the unitpower scales the weaponModifier of both sides when two regiments fight and it is the difference between those two MODIFIED weaponModifiers from tech that determines the impact of weaponModifier on the damage calculations, not the difference between the base weaponModifiers from tech.
I tend to go ballistic when somebody writes nonsense like Grisamentum's "Quality does absolutely nothing if you are at the same tech as your opponent." (meaning the unitpower ideas in Quality), because it as an assertion that can be disproved by the most simple of tests. You can use any country setup to trivially disprove this by testing how the unitpower/combat ability/weaponmodifier modifier actually works.
My own test when I checked 1.2 combat mechanics back in the day was simple - I assigned Burgundy a +400% infantry and cavalry unit power:
Code:
start = {
merc_maintenance_modifier = -0.25
cavalry_power = 4.00
infantry_power = 4.00
}
Then I started a sequence of 1.2 games at the 1444 start where I split 2 infantry and 1 cavalry regiment, put the prince in charge, and attacked Liege, which it just so happens has a 2 infantry 1 cavalry regiment defending under its monarch, which means that the regiments get matched up perfectly IIC opposite IIC and looked at the two die rolls in the very first combat phase and the resulting casualties. So you've got identical tech, identical regiment types, and identical lineup - all that differs is the unit power and the modified die rolls. As soon as you've done this 4-5 times, you are likely to have seen at least one case where the very first modified die roll for one side (in one test) is the same as the modified die roll for the other side (probably in another test) - noticing that the side with the unitpower advantage inflicts more damage for the same modified initial die roll, you conclude that it affects the damage. End of story. Further tests are/were of course needed to examine the scale of impact of unitpower.
But the end of the story is simple - in 1.2 unitpower (from Quality or elsewhere) helps you throughout the entire game, regardless of your tech level and regardless of whether you are fighting opponents of equal techlevel to yourself or not. Throw that on top of providing more discipline than Offensive does and working for all armies, navies, and garrisons rather than getting bonuses to leaders like Offensive does, and Quality is in 1.2 king of the combat arts. (Just as it was in 1.1, 1.0, and in 1.3)
Now, if people merely wanted to go and run wild because they thought the usefulness of unitpower in 1.2 was too low or the way it affected non-intuitive, that would be one thing and one I could get behind, but ever since anomalacaris began testing this in public based on his incorrect assumptions (that he to his credit corrected once he realized he was wrong, though he'd have had a considerably easier time of it if he tested fixed setups of nation vs nation rather than rebels, as rebels are notoriously screwy in implementation

), the forum has been overrun by people who misunderstood his results or drew erroneous conclusions like Grisamentum here.