Adding and subtracting numbers of wildly different magnitudes is always cursed. If you're doing this I'd say the majority of the time you are making an error.
Your equation ends up being like "We calculated the diameter of the observable universe, but what if we added the Empire State Building to one side, how does that affect the result?"
Your equation ends up being like "We calculated the diameter of the observable universe, but what if we added the Empire State Building to one side, how does that affect the result?"