CAGs have never in any version of HoI3 I've played had 5% stacking per airwing. (Regardless of how much I want and have tried to mod the game to work that way).
It was the normal 10% in 1.0 and then changed to 2.5% for certain missions in SF.
For air intercept/port strikes they use the normal stacking though and I have never claimed anything else.
What you have been seeing with CAG-duy is not auto-intercept, just the fact that they join other ongoing air battles in the same seazone they operate in. If you don't have other airbattles but intercept enemy planes somewhere else this isn't a problem.
Sure enough, you're right - I did a naval strike and 2.5% stacking penalty. IDK why I miscalculated when I checked the penalty before. A 2.5% stacking penalty changes everything for me. What's going to survive repeated strikes by 20 CAGs? Since the impact of positioning on CVs is unclear maybe an all CV fleet is the way to go. Create 12 CV fleets, send 4 CAGs for air-superiority or intercept over the fleet and 20 out on strike. Add the fact that the impact of positioning on CVs in uncertain (at least to me) and the rules are even more favored towards carriers. JMO, but a 5% stacking penalty for naval strikes seems a lot more reasonable and balanced.
FWIW, I go the 5% figure from the WIKI, and while I know that it's not current with FTM for some reason, there's a lot of rules FTM didn't changes. The problem for me is that because I bought HOI 3 Collections I don't have ANY change logs for FTM or SF - only the vanilla v.1.4 change log along with the manuals. I tried searching the forum for prior patch and expansion change logs, but all I've seen is the one for FTM v3.04. Paradox needs keep the WIKI game mechanics section updated (official site or not - it can't take that much time), have a forum sticky thread listing the current changes, or at the very least keep all the prior patch and expansion logs so those who bought HOI3 Collections can at least have someway to know what exactly are the current rules. SOMETHING so that there is ONE source players can go to for up to date, accurate rule information. Sorry for the rant, but it's incredibly frustrating that because I bought HOI3 Collections, I have to spend so much time just to determine the current rules, and despite my efforts discover that I missed something which markedly impacts my game strategy. It's like I'm being penalized for buying the game and expansions at a discount.
You said you tried to modify the stacking rules. I tried to find the mechanics rules in the game files so I would know what they are but never found them - where are they located? Also, would you please explain the "poor screening" penalty. I know it's when the ratio of capital ships to screens is below a certain point (I think it's 1 to 1), but what is the penalty exactly? I'd like to know so I can make an informed decision as to how many escorts I want with my CVs.
As for the non-naval strike CAG penalties, I know you never claimed that reduced stacking rules apply and did not mean to imply so. As for the "auto intercept issue," that's what happened. I may not have used the right term, but my issue is with the result. If I have nearby INT, I uses those for INT/Air-Superiority missions and leave my CAGs resting to jump any passing enemy fleets without losing organization or strength. Clearly, it's a good thing to have them engage enemy planes to defend the CVs, but if there's already an airwing providing defense, it's counter-productive. There's should be some sort of adjustment made so that they don't join a fight already in progress.