Jab will clear the people to Marth's outer area, as shaya has stated.
Here is my move to move reference.
Glance hit (shadow hit) of a Dtilt means you're perfectly spaced for a tipped Fsmash
Glance hit of an Fsmash means you're spaced for a tipped (super tipped) shieldbreaker
When opponents dash or approach (and penetrate Fsmash's zoning ability) Dsmash will generally tip where an opponenent "just got too close" for a tipped Fsmash.
Sooner or later, you'll be able to eyeball where the tip end up to be. FYI, tip hitboxes are slightly disjointed. Don't take my "move to move" comparison literally. Don't use dtilt to space an Fsmash etc.
...
Aerial spacing is earned through practice.
Retreating Fairs earn you a tip when your opponents run into you. But, seriously just try to weave close enough but not close enough. your mind set should be more to tipping than making contact. Once you've got your spacing//zoning done you can implement other strategies.
Such as, an untipped falling fair -> Dtilt -> Fair -> dtilt chain.
Or dtilt (untipped or tipped) -> DB
Practice. Practice. Practice!
And if you would fight CPU's fight level 3. Allow yourself to 3 stock them. And earn the least amount of damage possible.
But the caveat is... the chains and strings you learn fighting level 3 aren't reliable against real people. Sometimes they are.
Just learn to react. and do that via practicing level 3s.