Note that crawl depth does not necessarily mean the minimum distance that someone would click to get from the starting URL. Also, people often arrive on fairly deep pages, and what you really want to know is how long it takes for the user to get to the page they need.
Note that lower level fields may sometimes be needed to compute more useful fields. Also, sometimes the higher level fields may be more difficult to compute, so they are not always worth it.
Crawl depth is automatically computed by Chimera, if Chimera does the crawling (although you lose some benefits, you can import data from other crawlers as well).