That makes sense and is what I would have expected - guessing my source may have just been speaking in lose terms (out of despair perhaps!) shall we say. I remember trying to make the point we needed a 'buffer zone' of x miles for each class per annum, but in reality both were right up against the mark for the most part, the 444's being the worst, hovering just under/just over most of the time, with a reliance on engineering work (and/or Control) to keep them under by the end of the financial year.
Not that I'm part of it anymore, but if only! Remembering my care free days diagramming SUB's, EPB's and the rest....