Some stuff is automatic hit confirms. Some of it is read dependent. Some of it is DI reactionary.
Short version for tech chasing spacies:
1. Grab can cover all options on reaction, provided they don't fall off the stage.
2. Knee and Stomp cover 2 options if timed well(Tech in place missed tech, tech in place tech roll towards edge)
3. Nair/Uair + Grab can cover 2 options (missed tech and either tech toward or tech away)
4. Side b can cover 3 out of 4 options if you are right next to them (tech away, tech in place, missed tech)
5. Flying stomp can cover extra options, but the timing is tight. You can cover missed tech, then land to cover the tech roll away.
6. Of course you can hard read stomp and knee to cover any 1 of the options.
7. At around a 120% up throw knee will kill regardless of DI.
8. Start U-throwing around the 70% mark, Bad DI leads to CG shenanigans, early Knees and possible F-Smash setups.
Something to think about: Falcon's combo percent and kill potential are directly linked to how much stage you have to juggle people over. Some times it's better to re grab until they give you the entire stage to combo them across.