Not using a PM, and weighing around 200lbs, I tend to estimate calories burned at 20 per km, 30 per mile. Obviously that isn't accurate on any given ride because of wide variations in intensity, but they balance out over the week and the big picture seems to work reasonably well. And fwiw, it usually comes in about 20%-25% lower than my garmin 500 tells me, so your .75 formula may work just as well.

Interestingly, Strava (presumably based on their average power estimates) is generally closer to my own estimation.

Back to dietary regimens, the 5:2 thing really works and takes a lot of the pain out of it. One only has to be strictly disciplined two days a week and there's no need for a journal. But I found it was strictly an off-season thing. As the training intensity builds it becomes more and more difficult to insert sub-600 kcal days without impinging on performance.