So what I've been doing research into is MALLC with different stages. You can use the stages as references in order to MALLC from high areas. There are advantages to this, such as being an easy way to utilize a specific art you want. Another is to cover landings after being hit high up. When I'm doing this, I'm using Bair. It has more hitstun than Fair and Nair, so it needs lower heights to work. The difference isn't too significant, though, and you should still get a decent MALLC with Fair and Nair. I think the combined hits of Dair and Uair have more hitstun, so they may not work with these heights. Single hits should work, though.
From this height, going at full (constant) fall speed, I will MALLC when I hit the ground. Any higher than this and the activation of the art will be before I land. At later times, there will still be a MALLC, but the lag increases.
Just imagine a horizontal line from the point of reference on Shulk's body to the part of the stage. Usually I'll either use Shulk's head, feet, or waist as reference points.
Battlefield
The reference to use is when Shulk's feet are just above the top platform. However, you still get a great MALLC if you use the top platform as reference instead (still, feet on top).
To MALLC onto the lower platforms, envision a third platform (with same vertical distance) being above the top platform. Think of it kind of like Big Battlefield, how you have the third platform on top there. Hit B when your feet are just above that imaginary platform. If you want to MALLC onto the top platform, you need to either imagine the height of two platforms, or just put the height from the ground to the top platform.
Town and City
This photo is a little off. Shulk's foot is supposed to be on the black line. Anyways, for this part of Town and City, you can either use *a little above the side platforms* as reference, or if the camera is zoomed out at this height (result of you being high in the air and opponent being on ground) you can use the cliff in the background as a reference.
For the other part of Town and City, the moving platforms are just a little above the height of Battlefield's shorter platforms. All you need to do is imagine twice the height of the moving platforms (or imagine another platform stacked on top of it). You may be able to use some things in the background as reference too. I wouldn't recommend it, though.
Smashville
The height of the moving platform on Smashville is the same height as the paltforms on the second part of Town and City. Use the same method.
Kongo Jungle 64
If landing on the outer parts of the base of the stage, Use when your foot is right by the higher part of the side platforms.
For MALLC onto the lower part of the stage, we must figure out the height difference in reference to Shulk.
The difference in height is from his foot to his waist. Therefore, to MALLC onto the lower platform, activate your art when your waist is on the top of the higher part of the side platforms.
Use your own judgement for when to activate the art if landing between the top and middle parts.
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These are the only stages I've looked into so far. After all this testing, though, I've kind of gotten a feel for the actual height from which to MALLC from regardless of stage. It's a little more than 2.5 Shulk heights from the ground to Shulk's feet. Or, if you're using Shulk's waist as reference instead of his feet, then it's a little more than 3 Shulk heights.