Question about Flex Watt

Is flex watt supposed to be installed on the back wall of the snake rack or should it be installed in the middle or on the end? Thanks!

The back wall is fine (back heat) and the front or back bottom is also fine (belly heat) the middle is not really great as it will not generate a good gradient.

- - - Updated - - -

Remember racks usually need correct ambient room temps to hold correct temps typically 80ºF.

Thank you. And should I drill holes in the tubs for ventilation?

Soldering irons/pens work best for making holes.
I keep my room at 73-75 degrees making the cool ends of my tubs about 78 degrees.