I am trying to develop a spreadsheet to calculate a convection coefficient with altitude as one of the inputs. What variables should I consider? The most obvious are pressure and temperature, but I'm sure there is more to it than that.
Your model for convective cooling depends on what you need it to do - cooling an avionics deck is, in many ways, a simpler problem than just modelling all the possible physics that varies with altitude that could also affect convective cooling.
The article starts with something of a worst-case scenario (the big list of possible properties to be accounted for) and simplifies down. Above, I was going the other way.
Your objective restricts what you need to account for.
There are also research papers and so on all in this field - how far you need to go depends on stuff only you can know.