Drive shaft measurement

At least any newb who reads this thread will understand that the posters on here are all very concerned about the same thing: It's a lot easier to get the wrong driveshaft than the right one. Someone without driveshaft experience (and who tends to oversimplify things) will end up spending extra money on a driveshaft that doesn't fit their car. But we can't expect a person who doesn't know what type of trans they have or even what type of rear end they have; to have the knowledge and experience enough to figure out where the center of their front u-joint would be if they have no driveshaft whatsoever. If the op measures from the trans rear seal to the back of the u-joint cup, he will begin to have some idea if some random driveshaft will be close enough to consider. The rear seal mark will be on the yoke; just measure from that wear line to the back of the u-joint to compare. Then there's the question of yoke splines...