Yeah, it's a simple dmg% reduction.
Nah it's not. Like I said I'm a bit rusty on my math. Intuitively I know that higher accuracy units are more affected by accuracy modifiers. High ROF units tend to have lower accuracy and aren't as affected. I don't remember enough of my math to explain why, but I think it's due to the asymptotic behavior of 1/x. Either that or some geometric/ trignometric bullshittery. IDK exactly, and I'm driving myself mad trying to remember.
Because I can't explain this as well as I want to I went ahead and fired up cheat mod.
*Using range 10 cause there's a "hidden" mechanic called point blank which makes small arm fire ignore all but garrison/negative cover modifiers at ranges below 10.
Thank you for mentioning this btw. I thought I was going crazy until I realized my first trials were too close.
First I did 10 trials of panzer grens vs conscripts. 5 in cover 5 in open ground.
15 of the 20 pgrens that fought in equal green cover remained.
12 of the 20 pgrens that fought in equal open cover remained.
15% difference in favor of equal green cover
https://imgur.com/TuSqGT4
After doing these trials, and then re-reading the thread I realized pgrens have focus fire= false, and I didn't want that spoiling everything. I did my next trials with Obers and thompson rangers. Picked due to the accuracy disparity, and the focus fire = true on thompsons.
A total of 30 trials. 10 in open, 10 in green, 10 in yellow.
From the open cover fights: 22/40 obers remain. Rangers lost all fights.
https://imgur.com/tTmrbAE
From the green cover fights: 21/40 obers remain. Rangers won one fight with 2 models remaining.
https://imgur.com/l7QzyGS
So the damage reduction from green cover tends to hide the disparate effects on accuracy. This is most likely why the devs chose to do damage reduction instead of yet another accuracy multiplier. We should expect to find the largest difference in yellow cover, and we do!
From the yellow cover fights: 18/40 obers remain. Rangers won one fight with 2 models remaining.
https://imgur.com/dqxBlvh
From these trials there's a 10% better performance from Thompson rangers fighting obers in equal yellow cover than in equal open cover. 2.5% in equal green cover. Keep in mind I hand selected these two units for the largest accuracy difference I could find. Most units won't experience such a large difference as their accuracy stats tend to be fairly close.
With all that done and said: no it's not a simple %dmg reduction.