Here is a method courtesy of FootbrakeJim to check the fluid level accuracy of your TH350 transmission dipstick. The method is endorsed by Eman as well.

The procedure may work on other transmissions as well?? It may be already be common knowledge among mechanics.

I copy and paste here:
"With pan off, dipstick in, you can see the end of the stick poking out of it's tube from under the car. Take note of where the marks are on the stick, in relation to the pan mounting flange at the bottom of the trans. Better yet, mark the stick right at the pan mounting surface. That is your MAX full fluid level. Mark it with a Sharpie, but after ward pull the stick out and make a permanent mark on the stick, (as in a notch or groove that won't disappear like the marker ink will. Oh, and that will be your HOT fluid level, so make a cold fill mark about 3/8 to a half inch below that. Trans fluid expands a lot when it is up to full hot-lap temp, so the level will rise about 3/8" with a stock pan and cooler. Bigger cooler and a deep pan, it will rise about 1/2" when hot."

"One more thing to make note of: Since your goal is to keep the fluid level below the pan gasket, and you check ATF while it is running, keep in mind the height of your trans cooler and lines. If the cooler is above the pan level, (I'd guess most are, at least on door cars), when you shut it off, that fluid will drain from the cooler down into the pan, and push your fluid level above the gasket and seals. So if your ride leaks while it is sitting, but not when it is running, that is probably why. If you keep the lines routed down low, (like the car manufacturer did), the cooler should hold that fluid and not drain out."

Thx again, FootbrakeJim!
Troy