My first reaction to the unassigned categories list was "94 books down!"
But then I had a better idea. One of the drawbacks of this contest is that I only get one book per category, no matter what. This means that sometimes I have to skip extremely interesting looking books because the category is already filled. Even though I've only read 6 books, I've already had a dozen or more blocked for this reason.
My solution is to use the unassigned categories as wildcards. For each unassigned category, I will read another book from the same class. For example, there are 7 unassigned categories throughout 600 Technology. That means I can read 7 more books from any category in 600.
Another advantage is that it makes the contest a nice round number. Instead of saying I'm reading 906 books, I can say I'm reading 1000. It also makes a lot of math simpler for things like progress bars and charts.
And before some rules lawyer out there brings it up, yes, I initially considered staying in the same subcategory for the substitution. For example, the replacement to 699 [Unassigned] must be something in 690 Buildings.
I rejected it because I don't think it will work very well. The reason there are unassigned numbers in a subcategory is that there simply aren't enough topics in that area to need numbers. So I'm probably not going to have very many duplicate books in that subcategory in the first place. In some cases, such as the 110 Metaphysics area, my library literally doesn't have ANY books, so finding a replacement in that category would be madness.
Of course, if you can't live with this egregious rule bending, you are more than welcome to start your own, far superior, Dewey reading contest. Please let me know so that I can link to your site.