I've never known where the year break is either. I tell folks that when it doubt put it in at the shallower setting and if it isn't deep enough it will be obvious and you can set it deeper. Alway helps to make note of how deep the original was in also.
"Between two explanations, choose the clearest; between two forms,
the most elementary; between two expressions, the shortest."
Between two motorcycles the simplest (DW)
Eugeni d' Ors (1881-1954); Spanish writer.