On my 2014 T&C, when I insert the oil dipstick into the transmission filler tube, the dipstick bottoms out at the bottom of the transmission pan. (After sitting overnight, the level appears to be right about at the crook of the oil dipstick, when the outside temperature is about 40 degrees F.)
The dipstick that one can purchase and use according to this procedure cited above (
62TE Transmission - Caravan's without Dipstick Refill - Bob Is The Oil Guy) appears to have a stop on it, such that it "bottoms out" at the top of the filler tube. (Note: this bobistheoilguy procedure is for a specific transmission, so for other transmissions the measurements in the chart are likely going to vary). From a quick look at the procedure, it looks to me like the special tool measures the true depth of the fluid in the transmission oil pan.
(Note the procedure includes going through the gears, etc.)
As can be seen in the chart in the procedure, there's a range of minimum and maximum for the transmission oil level. Depending on temperature, this range is anywhere from around 12 to 16 mm, or so. The minimum temperature in the chart is 70 deg F, so we won't have any data for temperatures below 70 deg F unless we do it ourselves. If we were to use the length of the filler tube, we could convert between the two dipstick methods (bottom out at the bottom of the pan, versus at the top of the filler tube), but there seems to be no need for that.
Here's a version of the dipstick hack from youtube:
.
I think it would be difficult to reproducibly get fluid levels for a transmission a couple of minutes after starting because the level will depend on the ambient/outdoor temperature. A couple of minutes after starting in Yellow Knife will result in a far different level than in Miami! Doing it at operating temperature should work fine, and then the chart ought to work using the special dipstick or using the oil stick if the conversion is known. Operating temperatures themselves are likely to vary with latitude and season.
It would be interesting to develop some data ourselves on factory-filled transmissions that have never had a leak. I'd think we'd want to do it on a transmission that has sat all night. Then we'd want to measure the (relative) dipstick level (say, using the oil dipstick) and the temperature. For temperature, we could use the outdoor temperature first thing in the morning. Of course, this would all vary with transmission type. Then we could get some averages and there you go. Or, given the difficulty of all that, we can just mark our dipsticks before things leak, so we're ready when they do, or when we do a fluid change-out.
I plan to change out my transmission fluid soon, although my T&C has about 45,000 miles. The car is new to me, and I like to change out all the fluids to start out. But I'll not plan to do the filter, as I've read that most times, transmission oil filters (on these Chryslers, anyway, but it likely applies to others) don't get munged up unless there's a mechanical breakdown.
'Nuff said for now!