Here's my view of Itachi's full power
He starts at 100% (which was exactly after obtaining the MS)
After several years, he comes back to the village and fights Kakashi. My guess is that Itachi needed to use MS a lot when fighting his own kind so he exhausted it. Then he had to use it quite often to prove himself worthy for the akatsuki and to fend off people like Oro, Deidara and I'm guessing those two aren't the only ones who regarded Itachi as annoying. So he should be at about 65-70% of full power when he fights kakashi.
After 3 more years he fights naruto and kakashi. We know by that point that his eye vision is aggravated. he should be around 45-50%.
And after a while he fights sasuke. We know that he's been living off medication and barely dragging his life when he fights sasuke, but nevertheless he is still capable. I would put him on 35%.
So overall, I am estimating Itachi's full power was almost 3x stronger than the Itachi that fought Sasuke.