the thumb i use for the space key is whichever one typed the last character in the preceding word.

I did take typing, and I do exactly what you do. I didn't know this, of course, until I sprained my dominant hand's thumb three weeks ago playing basketball.

Spraining or breaking fingers wreaks havok on typists (which most of us are), but it surely puts me out of commission from doing my sign language interpreting. Up until a few years ago when I became a staff employee at a USn Federal agency, I worked *freelance. Back then, it was unpaid unemployment if I ever hurt my hands (or wanted a vacation, or took a day off, etc.).