The whole objective of any route is that you complete the distance. Calendars need to have a fixed start/finish, for the issue of Brevet Cards, if for no other reason. Not needed if the Brevet is posted to you, so if you live on or close to the route, just find a good ATM/Garage to prove your locations/times and you are off. When in Eire last May, I took a photo of my saddle with my watch/HRM on top with a road sign in the picky; on return I repeated the picky (24hr clock mode). Pics are also good for info Controls (I use them for my own Perms (‘cos I know the answer) and when I needed to prove I was on Trike).
Note that on my Thames and Avon 200 Hungerford would make a ideal Start/Finish as the route links Fairford, Hungerford and Corsham with Cheltenham. It is a good winter route with good road alternatives too avoid some lanes.
One-way systems can upset the flow of a ride, if your hit the ‘wrong’ way. This is why an issued routesheet, based on an Organiser recce can help. I find Stroud can be awkward E-W as the natural flow is W-E. N-S or S-N is not a problem, though you have to divert if you need a Control.
Only an unforeseen diversion (flooding/road closure) can be used to claim extra distance, so you forfeit extra kms if you join from off route. You will be timed on the distance declared by the Organiser, which will normally not be exactly 200km; so a 214km perm will gain an extra 1hr above a 200km but if it takes you 230km you lose 16km (which means you lose an hour, so you need to you can afford that loss.
SteveP