Pistongone...
The point you're missing is the sums I'm using to get myself accross the channel are based on my inflight experience NOT what I do on the ground. Also do you arrange your flight so that your groundspeed is a convenient factor of 60? Or do you fly at 2350rpm etc etc however your POH tells you to fly an aeroplane?
The point I was making was I think in my opinion that people spend to much time working out the numbers and basic their assumptions on other peoples' guessing (ie forecast winds aloft). Becasue of that I prefer to have a route on a chart and estimate my headings and timings dependant on what I think is right at the time. And THAT gets me accross the channel not working out the numbers beforehand.
As I say if your method works for you then that's grand.. mine works for me!
Bottom line is.. GPS rocks!