Sorry to say but I dont' own one (yet, maybe someday) but I got sort of rule of thumb for ya.
And a link about
Boss DM-2 for anyone else interested.
For rockabilly stuff I start like this, usually:
Length I like is somewhere about 110 - 113 milliseconds.
Since the DM2 doesn't have numbers you could use some other digital pedal that has and compare by ear.
Also, listen to songs like BSO "This Old House" and in the beginning of that song
there's a pretty cool delay pops to compare for.
Something like 3 audible pops is enough. (about 30 - 35 % in digital)
Then the mix od delay and pure signal depends on the song. Some songs wants more than others.
Currently Im using digital stuff so for me it's easy to set up but
for analog pedals, it's about using your ears.
Hope this helps.