I don't use Lilly's terms, I use the Egyptian ones, and in those, Mercury's term in Cancer ends at 19 degrees.
There is some controversy over Ptolemaic terms, as you know. Deb Houlding did a wonderful article, as well as an experimental table of Ptolemaic terms gleaned from the knowledge we have now compared to Lilly's table - and Mercury in Cancer is one of the terms that's in dispute by a couple of degrees.
Find Deb's article - with tables - here:
http://www.skyscript.co.uk/terms.html
It's a good read, but for your situation it's splitting hairs. Any essential dignity is enough to keep a planet from being peregrine, and Mercury definitely has face at 19 Cancer. So it's not peregrine.