In computing the relative humidity of air after it has warmed, I need to account for the change in volume. By Charles’ Law, given stable pressure, volume is proportional to absolute temperature (i.e. V=Tk, where K is a constant). However, the sources I have read do not give that constant, for air. But seeing what I’m interested in is that amount of air which is contained in a cubic metre at standard temperature and pressure, then I think I can just take (temperature-kelvin/celsius-offset) as the temperature component.

Gay-Lussac’s law says pretty much the same for pressure, so again I think I can take (pressure-pascals/Pa) as the pressure component.