Fair nair works at %s like 120-130%, but before that your opponent can DI up and jump out since double jumps come out on frame 1. Fair doesn't combo or string very well until high %s which is why I normally just use it as a spacing tool or I'll use it to the moves completion. I fall a lot more with dair than fair (On dat #YOLO strat)
Standard landing lag is 2 frames, btw, unless you are FFing or air dodging, in which case it is 4 (Called soft and hard landings, respectively) which is why it is easier to punish somebody landing with an airdodge as opposed to somebody just landing. You would be surprised how much 2 frames make considering our charge release on F-smash comes out in 1 or 2 (Don't remember).
Also, y'all can argue amongst yourselves, just don't argue with me about MUs that I understand. You can argue with me about Peach/Olimar like it's your job since I don't actually know them too well....just don't talk about MK/Snake/ZSS lol