One thing people never comment on is how the planners knew all the routes would have sufficient fuel. Well the planner at Bomber Command used a very simple rule.
First, all the routes were planned on a 3m chart. Major positions on the route might be a 100 mile arc based on Flamborough Head, then tracks would fly parallel to a straight line position, such as the Go Line before flying dense parallel tracks toward a few chosen penetrations points. Aircraft would be planned to fly through penetration points perhaps 4-5 miles wide in as short a period as possible, say 15-20 minutes.
Once through the coastal SAM screen and out into the hinterland the force would disperse although some notional corridors were used. Aircraft would join the corridor at some point and others would leave at others. It was carefully designed to suggest that a couple of aircraft were flying in a particular direction towards a particular target.
Now for fuel. I took all the crew planned fuel and timings down to Bomber once. With one glance they were rejected as rubbish. Back home, closer analysis revealled gross errors, usually by the copilots. The 7000lb bomb had been used as fuel; the 325k low level had been fuel planned at 250k but 325k timing; wrong graphs; gross errors; planned descent at destination etc.
How had our man at Bomber known they were wrong as he only saw a one page summary?
His rule of thumb was to take the fuel used from take-off to top of descent, from top of descent to final climb point. From final climb point to over head recovery base. He would multiple the low level portion by 3, add them together and reach a figure of about 5100 miles. This would be compared against the Vulcan high level fuel graph and the maximum distance that could be flown in high level cruise from full to empty.
Determined not to be caught out again I made a simple transparency with climb, cruise, low level, and descent segments on one sheet. When the copilots were not looking I would simply lay my graph on theirs. Immediately it would show all the errors, even a 10k or one hour miss plot. The errors were invariably in the crew favour; if the misplot showed mission impossible the crew would usually pick that up.