Back in the day most mechanics that worked on stovebolts had a tool with that had a screwdriver built into the wrench. You could put the box end over the lock nut, crack it loose and then use your fingers to turn the knob for the screwdriver and then use your other had to run the feeler gauge. They always adjusted the valves with the motor hot and running. It also helps to really slow the idle down when you are adjusting the valves. After doing a few it gets easier. Just take your time.