Pocket with a T-slot bit

I’m new here and haven’t actually built mine yet…so bear with me…

Is getting the spindle perpendicular with the work surface what’s important first? Or is getting it perpendicular with the X gantry what matters?

With a 3D printer, I am usually trying to get my bed(work surface) parallel with my X because there is no re-surfacing, but my understanding is that the surfacing of the spoil board will take care of making the work surface parallel to the X/perpendicular to the spindle…

So prior to surfacing the spoil board, should the goal be to make the spindle perpendicular to the X plane of travel, which in turn should make the bottom surface of the tool parallel to the X?

Then re-surfacing the spoil board would then fix the work surface to be smooth with no lines, as well as parallel to X and perpendicular to the spindle as well?

I’m just trying to make sure I understand correctly so that when it comes time to do mine I will know why I’m doing what I’m doing.