I follow the instructions in the owner’s manual on my '84 T-Bird for this. I just drove back from Boyle Heights, Los Angeles, to El Camino College in north Torrance–about 19 miles. Just after I parked, and with the engine still running, I checked the level of fluid on the dipstick, according to the instructions. However, when I wiped the dipstick, reinserted it, and pulled it out a second time, the ATF on it was along one edge, instead of coating the entire end of the stick. (This reached way up into the crosshatched area which indicates an adequate level of fluid after the engine has been running a while. (I’d been having trouble with the transmission shifting into and out of a gear at certain speeds and suspected the fluid level may be low.)
How do I do this to get an accurate reading? Or perhaps the dipstick or the transmission has reached a point where it can no longer give an accurate reading? 
Your fluid level is not even up to the end of the dipstick. Add fluid! The fluid on one side of it is just coming from the inside of the dipstick tube.