However, I should have calibrated that as 30%, as I knew the century but had no idea when in the century he published the book.

Yes! I made the same mistake.

If you know the century, there are only about (10/3) mutually-exclusive 30-year periods. Thus, the lowest your maximum probability out of all 30-year periods should be about 30%, and the one that you actually guessed should be at least a little higher than that. (of course, if your guess is within 15 years of the century boundaries, some of that probability mass is going to get splinched).