How use dipstick tool on 722.623 Trans ???

[Panchoz3]

How use dipstick tool on 722.623 Trans ???

Doing trans fluid change & flush on trans.
Popped off red tamper cap and inserted dipstick to get initial reading.
Dipstick stopped at 17 inches, showed no fluid at all.
Pushed a bit, and dipstick seemed to slip past a notch,
then went in for max of 27 inches, and showed over full
about an inch past the plastic dipstick end up onto the coiled body of dipstick.

So, either my trans. was so low that it did not show at the 18 inch length,
or, trans was so overfull that is showed at the 27 inch length.

Bought vehicle new in 2000, has 100,000 miles. Have never put fluid in, ever.

So, how do I use this dipstick tool, or more specifically, at what length
does on read the dipstick ?