Jessfix,
Thom 5 has a tiny paragraph on 'dip' but calls it "Turning Error" and defines as the loop taking time to resolve the signal.
BGS notes define dip as being something related to some Bendix King installations and is at its maximum with relative bearings +/- 45 and 135 deg.
RANT defines ADF dip as when the sense antenna shields the loop antenna. Says dip error reduces eventually to zero as the beacon moves abeam.
Take your pick. Guesstimating 10 degress with a rate 1 turn seems to work well enough every time.
I recall reading somewhere, that some Bendix King installations have been modified to remove the problem.