As others have intimated, there's nothing wrong with getting off track - that's the *normal* state of affairs! The problems come with not recognising the off track position in a reasonable time, fixing position and correcting back to track.
I too have a calibrated thumb. My right** thumb when bent at 90 deg is 20 nm on 1:1mil chart from tip to first joint when bent. 10nm from tip to nail root. Damned convenient of Lambert & his cohorts to make their maps to suit my thumb!
**Strangely, not my left. That one is a mile or two out.