What I hoped (still hope) to do is to ride both before and after a Calendar ride, mandatory routes, perhaps of different lengths so I could stack the "before" (I like early starts). Figuring out what the rules are re. cutoffs completely defeated me,
All easy enough - and the fact that it's mandatory route shouldn't make much difference, though the mandatory route statement in the FAQ is a bit woolly.
Plan your route - front loaded, back heavy, it doesn't matter, and send Martin a link to it - RWGPS, GPXEditor, whatever your preferred site is, along with your ECE entry. Ride. Send him your GPX and, if necessary, proof of completing the calendar event.
That should be it.
As for time limits, you get the actual length of your ECE, plus the nominal length of the calendar event, all at 14.3kph (or less if you're going over 699km).
Say you're in need of a 400, booked to ride a somewhat overlength 200 of 222km, and wanted to both front load it and get in some extra bonus miles, so were going to ride 135km before, and 90km home. You'd get (135+200+90km)/14.3kph, which I make 29 hrs 43 mins.
Conveniently, despite being well overlength at 447km, that's almost exactly 15kph overall. Start time may be left as an exercise for the reader ...