Hi
Congrats . He is adorable.
If you have stick-on type gauges- Then yes , middle height is fine. ( against the glass ...well...you have not choice)
the stick on ones are really the best, nor are they accurate.
Against the glass is not the best place and you typically want one in more than one spot or be able to move it around.

Consider a closed probe type only hygrometer- and drape the wire toward the middle. Plus, it can easily be move around. Seriously consider more than one thermometer as you will want to measure the heat gradients.

Below - These have a little magnet and can be attached to a leaf- my favorites N0 batteries, and the constant moisture never mess hem up !

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JW Pet Fusion Smart Temp Thermometer Magnetized
OR
These are great , cheap, accurate, and easy to move around.
Hagen Thermometers


http://www.frogforum.net/vivarium-te...-probe-go.html

Lynn