Advice for thermostat location for one 4ton unit heating both 1st floor and basement

I have a single 4 ton unit currently heating and cooling the first floor of my house. The basement is now being finished and I have been told by a reputable company that the current 4 ton system is the correct size to heat and cool the basement as well. He recommended adding a new intake in the basement and then cut in the new flex duct to the existing trunk line. He said it looked like the original heat and air guy sized the system for this purpose in mind.

Thoughts on this? Should the thermostat be relocated somewhere else?

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