As Shoreham is 17.83 minutes West I simply count 17 of the dashes to the left and .83 is just a finer, more accurate position between the 17 and 18 marker.
17.83 minutes = 17 minutes and 60 * .83 = 49.8 seconds. But you knew that, right?
Anyway, by definition one minute of latitude (note: NOT longitude) is 1 nm so if you only get the degrees and minutes right you can't be more than a nm off.