Measure the outside of the tube from the top to the top of the trans case next to he tube entry point - following the contour of the tube. Then measure the stick from the bottom of the dipstick cap that rests on the tube down the same distance you got from the first measurement. Mark the stick there.

Then warm the trans up, run it through R, low then back to N then check the fluid level.

If above the new mark drain until on the new mark you made. That should get you very close.