From: Romford, England
Well it depends
I usually set my CV CAP when playing the allies to about 70%. But it depends on how many CVs I have in the hex (usually I keep each CV in its own TF but have multiple CV TFs in one hex - they combine CAPs). This is because I value the CV more than the bomber squadrons on it. If I was going up against the IJ CVs then I might lower this level. I don't mind losing mine to kill his! But you can so rarely guarantee a fair fight and there is little worse than seeing a Datai of Betty's flying through and nailing a CV just because your Wildcats are hoping to punch a path through for your Dauntlesses. So usually even if expecting a fight with the
You cannot guarantee that your strike will get through the big CAP over Kido Butai, nor that the Wildcats will even escort your main strike. Plus the Japanese are deadly to the US CVs at range 5 (250KG bombs will sink US CVs) while your bombers can't reach his. Even with the Avengers having range 5 their bombs at that range (500lbers) will not hurt Kaga, Akagi, Shokaku or Zuikaku. So I focus firstly on defence. Besides CAPs, even large ones, get tired and become less and less effective with each strike. Even if they still have the same strength.
By your profile you are fairly new. So be aware that CAP is one of the 'issues' many of us have with WitP.
In real life CVs could not put a 70% up except for a very short period. Certainly not early on in the war when radar was simply not reliable enough and you needed to keep planes up at all times. In practice they would have about a third or a quarter up with the next third / quarter ready to replace them when they needed. The rest were likely to be held back ready to escort a strike. But in WitP you get all the ones you ask for - all day. There is no halving or thirding of CAP to get a more realistic figure.
This is compounded by another WitP phenomena - that air combat becomes far more deadly as the numbers get bigger. So for CAP the more you have the better it gets until eventually it becomes so good nothing gets through at all. The 'UBER' CAP phenomena. Basically a CAP of 30 is more than twice as good as 15, and 60 is much more than twice as good as one of 30. Rather than diminishing returns we have the opposite. Above a certain number an Uber CAP is likely to kill every strike aircraft, unless overwhelmed - when just about every bomber then gets through. But one side is highly likely to be wiped out. WitP does not handle large air combats well. They are far far too deadly.
So if you (and your opponent - AI or human) masses CVs you are likely to get Uber CAP and mass air combats. These give highly unrealistic results. Although they can keep those flattops safe
Allies vs Belphegor Jul 43 2.5:2.5 in CVs
Allies vs Drex Mar 43 0.5:3 down in CVs
Japan vs LtFghtr Jun 42 3:2 down in CVs
Allies vs LtFghtr Mar 42 0:1 down in CVs
(SEAC, China) in 3v3 Apr 42
Allies vs Mogami Mar 42 0:1 down in CVs