On my flight computer, I use the hardcoded launch site altitude and the current barometric pressure to determine the sea level barometric pressure with the formula:

Current pressure = Sea level pressure / [(273+Temperature-0.0065*Altitude)/(273+Temperature)]^5.256

Then, with the known sea level pressure, I get the actual vehicle altitude during the flight with the backwards formula:

Altitude = -(273+Temperature) / 0.0065 * [(Current pressure/Sea level pressure)^0.190263 - 1]

, where many of the hardcoded numbers are coming from different parameters of the standard atmosphere model like the temperature lapse rate, universal gas constant, molar mass, etc.

What temperature to use in my calculations?

My barometric sensor also measures temperature, but it is the electronics temperature, not the environment.

If I am using the standard atmosphere model's 15C, then someone might legitimately argue why I'm not also using the standard atmosphere model's sea level pressure.