How About, a = 60 x d / D, where, a = angle off track, d = distance off track, D = distance travelled (from station or last known fix)
so if the distance travelled was 35nm and distance off track was 7nm, then the angle off track is 60 x 7 / 35 = 12 degree's off track
And maybe I missed it! did someone explain that the 1 in 60 rule, is of course the rule of thumb! that if you have travelled 60nm and are 1nm off track that you will have a track error of approximately 1 degree. Or to put the other way if you were to travel 60nm 1 degree off track you should find yourself approximately 1nm off track!
And yes I see Farri has explained this, Oh Well !