Correct. Facilities also contribute to EM, and population also contributes to thermal.
However, since population is usually the leading EM contributor and industry is usually the leading thermal contributor most people assume EM/TH is proportional to population/industry as a quick guess.
The best guess I've come up with is: [EM - TH] / 45 = Population in millions.
This is always true if a colony has only population/infrastructure/mines/factories, but gains errors whenever shipyards, research labs, and all the other stuff starts to appear.