It gets tricky for the reasons you're saying, and you're correct that .850 plus-minus 0.01 could be misinterpreted as 0.015....but if you want to avoid it then just make all the callouts use three decimal places. Some drafting purist from 1995 might get angry, but the correct information would still be conveyed so who cares. Any specific dimension with a tolerance associated with it will automatically override the general tolerance information so you can make it whatever you want. The only thing is it might get confusing if you have a lot of tolerance windows all over the board, but hey at least everything is right there in plain view...