ET-_^ left when his team had lost one lane and were just about to lose the second, meaning the game was soon to be finished. Each lane lost subtracts around 10 days or so from the from the total penalty is what I usually go with, to a minimum of one day of course. In your case, the game might have been "decided", but timewise there was much game left. Though the scores were fairly even aside from Scourge's advantage in tower kills. Farm is of course a factor that can't be seen in the screenshot.
But I agree that you did not ragequit. I just wanted you to understand that we make little difference between motives for leaving. Leave because you are doing bad, leave because your team are doing bad, leave because the other players are treating you with disrespect, it's leaving all the same.
But you seem to have understood how to act in this kind of situation.
Penalty reduced to 10 days from now.