You have a 25% chance of getting it right no matter what. Then, tolerances being what they are, luck plays a role even if you get it wrong. But Ford put the paint dots there for a reason, and that's all there is to it. It takes 3 seconds to wipe the flange off when installing the driveshaft to find the dab on the pinion flange and line the dot on the DS up with it and eliminate the variable.