We're offroad twice a week by ourselves. Because of this, my Jeep maintenance is obsessive. It's always bothered me that I could not check my own trans fluid level without the dealer or a buying a special tool.
With the assistance of a friend who happens to be the lead transmission mechanic at my local Jeep dealer, here is a simple method we came up with. This involves writing on your Jeep, but works just as well if you draw onto a wall or work bench. I chose to write on my Jeep to keep the scale handy. If you choose otherwise, at least the measurements are here for you.
Junkyard dipstick at least 40" in length
Assorted Sharpie pens (Black, Blue, Red)
From the top of the dipstick tube to the bottom of the pan is roughly 38", so make sure you get one at least that length plus inches for a handle. Longer is fine since it won't be left in the Jeep while driving. The dipstick I picked up was from a fullsize van and measured 54". Assuming you've already picked up your junkyard dipstick, we'll continue from there.
Locate the body panel between the hood strut and radiator overflow reservoir as shown below. Place the metric ruler at the edge of that pinch weld and tape it in place.
Using a black Sharpie pen, mark in 10mm increments beginning at 20mm and ending at 70mm. Use your blue Sharpie pen to mark half a centimeter in either direction of the 20mm mark. Use your red Sharpie pen to mark half a centimeter in either direction of the 70mm mark. Remove the ruler. The result should look similar to the image below.
Remove the transmission fill cap and insert your junkyard dipstick and feel for it to bottom in the pan. Mine bottomed right at around 38", but you should measure your own length. Using pliers, I added half an inch and placed a series of bends in the dipstick for reference as shown below.
Drive until the vehicle is at normal operating temp. Park on a level surface, but leave the vehicle running. Lift hood, remove trans fill cap. Clean and insert the dipstick, bottom it and remove. Place the end of the disptick to the end of the pinch weld panel and compare to the chart you've created as shown below.
As you have noticed, no temperature readings were used or needed. The Jeep transmission mechanic stated that the acceptable fluid range for a properly working NAG1 transmission at normal operating temperatures will always be within the red limit.
He went on to say that ambient temperatures do not factor in much at all and a temp measurement for these purposes in not needed. Normal operating trans temperature is not affected by ambient temps. Ambient temps affect the trans by increasing or delaying the time it takes to reach operating temperature.
However, he did say when checking the fluid of a cold trans on a hot day, you should expect the level to read high side blue or in excess. When changing fluid, fill to just short of the blue level, drive to operating temperatures, and recheck against the red.
This method assumes that you have a healthy NAG1 transmission. If your transmission runs in excess of normal operating temps, there may be variations. Go to your dealer one last time and compare their results to yours to be sure. His final recommendation was to install a trans temp gauge to warn me of transmission issues before they become bigger problems.
One thing that he did find amusing was that some people really believe that the NAG1 is extremely sensitive to overfilling and that as little as a few ounces of overfill will destroy it. He said that people are repeating on forums and websites the technical releases about overfill sensitivity. That is just something dealers tell people to explain why it did not come with a dipstick. They were offering the lifetime powertrain warranty at the time and needed a reason for owners to come in. To further make this point, he said other vehicles using the same transmission that did not offer that warranty came with a dipstick. What he finds most amusing is that of all the crap people hear from dealers, that is one of the only things people choose to believe.
In his factory service training course specific to the NAG1 transmission, they were told that it does have an optimum operating level, but a well sealed NAG1 can safely hold and operate with an additional 2+ quarts without harm, but of course don't do that. If ever someone were to overfill to a dangerous level, it would overflow from the fill tube or burst the plastic couplers before any serious or expensive damage is done.
I've read many times that as little as 6 ounces of overfill can damage the NAG1. The NAG1 was used in many vehicles. Surely, if it were that sensitive, people would be talking about overfill damage. There is plenty of forgiveness in overfill, but stay as close as possible to the recommended level. The NAG1 mated to the Hemi motors. It's not that dainty.
Hope this helps.