The old fashioned way....I just tweaked until it sounded right.

I have a trick for finding mod speeds. I know I like sound up there
in the 3-4Hz range. So I instantly go there and find the frequency
where it "syncs" up with the delay. At that speed the mod depth has
to be turned way up for you to hear the modulation. I then increase
the mod speed beyond that point so that it gets out of sync with the
delay time just a bit. Adjust mod depth to taste.

/jason/