This TPR valve has a 4” thermostat element (probe). Seems to me the probe might dip into the water ½” to ¾”. My understanding is that the probe should be “immersed” AT the top 6” of the tank but I can find nothing about exactly how far the probe should go into the tank. Is there a generally accepted rule as to how far these probes should go into the tank to be considered “immersed”? I believe the shortest probe for a TPR valve is 2” so, what, maybe at least 1 ½” into the tank? Or maybe half the length of the probe?
The valve must be installed directly into the tank or hot water line.See the installation instructions for your pressure relief valve (100XL).
https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/55926-source/tag-100
The tape measure has fractions printed on it! What’ll they think of next? 
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You do know most kids today don’t know how to read an analog clock or watch. There was a time each generation was more educated than the prior. Not anymore.
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The tape measure threw me off so much that I missed the TPR valve!
Kuddos to the tape measuring company. They recognized the impact of removing shop class or other Vocational studies from schools and are trying their best to give a young person a chance.
(PRVs) are a most important water heater line of defense against equipment damage, not to mention physical harm.
Most safety valves are designed, manufactured, and tested to be used vertically, in the upright position with the spindle vertical. Mounting valves horizontally can have a number of significant adverse effects.
In your case, the stem is immersed in water. I would note the white residue between the fitting and Valve.
Do you have a picture of the entire top of the tank? Why is flexible copper tubing connected to the TPRV? something seems very very wrong here.