Genghis,
Re the 6-minute marks... one advantage of them over other methods, I find, is that they don't need to remain 6-minute marks for the whole flight. For example, if you find yourself flying slower than planned for whatever reason, they suddenly become 7-minute marks. This makes it extremely easy to calculate new ETAs, even when you're not at a convenient point such as the half-way point of you leg. Just my personal preference, of course.... this is a very individual thing.
FFF
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