So Sheik's fair has 8 frames of landing lag when L cancelled, and does 7 frames of shield stun, so you calculate that if you are perfect, this move has -1 frames of lag.
Is it possible to hit the fair on the same frame you land? Don't you have to account for one extra frame to hit the ground?
i.e. you hit with fair on frame 7, touch the ground on frame 8, then suffer 8 frames of landing lag. You lag until frame 16. Meanwhile, you hit fair on frame 7, they suffered 7 frames of landing lag, and they lagged until frame 14. You had a -2 advantage.
Or am I mistaken?
Shield stun doesn't begin until hitlag ends. The frame after hitlag ends, assuming you fair low enough, Sheik is in the L-cancel landing lag animation. Starting from the last frame of hitlag, it would look like this:
Frame - Sheik - Shielding Opponent
0 - Fair hitlag - Fair hitlag
1 - Landing lag - Shield stun
2 - Landing lag - Shield stun
3 - Landing lag - Shield stun
4 - Landing lag - Shield stun
5 - Landing lag - Shield stun
6 - Landing lag - Shield stun
7 - Landing lag - Shield stun
8 - Landing lag - Free to act
9 - Free to act - Free to act