@ BS,
In the little patch that I trundle my wheelbarrow around, I only know of one 400 m line where the JDSU TDR trace resembles the worst white knuckle ride I can imagine.
An exceedingly good Openreach engineer looked decidedly sheepish (Sorry BS !) saying that each of many cable joins showed the normal distance-based degradation.
That EU played merry hell with BT accounts and was reimbursed but had to accept the standard 10 / 40 contract instead.
Otherwise BS's 400 m rule seems spot on.
Kind regards,
Walter