It really depends. Some MTTs, I might only about 15% - 17%, but in others, I've been around 24% and I know there was at least a couple where I was near or +30%.
You cannot play good poker by playing according to pre-determined hand charts. You must learn how to adjust your ranges based on all of the factors presented to you. There are times where you can find the same hand in the same situation and one time it will be a fold and in another it will be a raise.
Take a hand like 75o from the cutoff. In general, this hand is not on any typical hand chart to play. So most people will by default fold it, because it is a trash hand. But if I am playing against tight blinds, I will make a steal attempt if my image is good.
But then again, hands like T9s from the same position is on a lot of standard hand charts to steal. But if the blinds are really good players who won't give up without putting me to tough decisions, I would fold it.