Actually, if it were just a straight E, it'd still be 10 times stronger than a normal human, but with the minus there, she might be... 8 times as strong? I'm not sure, I'm not really sure how the minuses work. But yeah, with Servants, E is ten times as strong as a normal human, D is twenty times, C is thirty times, etc. So he is right there.
I do still feel some of her abilities don't match up to those stats, however. She may be about 8 times stronger than a normal human at everything, but I still stand by the fact that it would be difficult for her to form a separate form from the castle with E- in magic, due to what I suspect would be a massive toll in terms of prana consumption.