I do woodworking a bit too, but I normally just do the slanty ruler/tape trick to divide any straight parallel face into n equal lengths.
I hate all forms of mental arithmetic; I also avoid measuring as much as possible too. Maybe that's why everything i make is so shit.

I guess if you're mass producing things you can't just manually mark off each and every part though - but even then I'd probably want to work to a template rather than to measure.