How would the temperature on earth be if the sun was slightly more or less powerful?
Don't have high enough reputation to comment, but someone should mention the Stefan-Boltzmann Law: https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law
When a planet (or anything) gets warmer, the amount of radiation it emits increases with the 4th power of temperature (measured on an absolute scale like Kelvins).
A planet's temperature reaches equilibrium when it receives about as much radiation as it emits back into space.
This should give you a better approximation than the linear model.