Hi.

I use 50-50% when the opponent has strong strike craft capacity (even higher ratio of fighters when I start building strike craft significanty later), if I am the superior one and I dont need to field large amounts of fighters, then I start to increase number of bomber squads. Against AI it was not unlikely for me to have even 80% bombers and only 20% fighters. Bombers in general are better against more things than fighters, a large bombers swarm can easily remove titans, structures, starbases, capital ships and even cruisers at ease.

I dont think there is a huge difference between races, same e ratio goes everywhere. If not, someone will correct me I guess later.

If your opponent has a large amount of corvettes and it is becoming annoying you should have more fighters to kill those easily.

For strategical structures (starbase, hangar bays) I tend to build more fighters, those are better I think for defense. They are good VS siege ships and bombers, and an assaulting fleet is very likely to have them.

Against humans I have no idea, haven't played multi in ages so I have no idea what composition is considered effective against a more skilled player.