Importance of abs in levers?

Very often I hear that most important part of levers (back/front/side/plache) is abs/lower back. I know that this is not true. Most important part is shoulders/back/pecs (upper body generally). But how much strength do you need in abs or lower back so that you can said: "That is enough, that will do"? Is it possible to make some kind of relation like: 3x30 sec of L-sit is more than enough of abs strength for front lever. Or maybe 10reps of legs lifts.... something like that?

All those people who says that abs are most important are lead by feeling that they get while they are trying to do certain lever. They say: "I can't hold my legs because my abs are not strong enough". So, if feelings can trick you, how can you for sure know where your problem is: abs or upper body?

I know that if I want something done, that I should just follow progression from F-series. But I am curious about this and I want to know how to prove someone wrong when they say that levers are about abs.

You are right  that the shoulder, upper back strength is the most challenging aspect and that the Foundation series lays it out perfectly.

 

Without getting into details it first addresses abs then shoulders. If you have it you will already have the answer to your ab question, just look at the last ab intensive progression, that's how strong your abs need to be.