Hi Michael,
It is possible to adjust the micro focus with some Canons; not sure if the 550D supports it (my 50D does). The link below gives the gist of it or search for "Autofocus Microadjustment".
https://contrastly.com/autofocus-microadjustment/
PS when I modded my 50D I didn't measure the screws either (d'oh), but when I reassembled I tightened the 3 so that the sensor board was resting on the pegs/stops to at least make the sensor as square as possible to the light path. Surprisingly my AF isn't knocked around much.
Cheers