10x strikes me as an appropriate factor since it gives the provider a strong disincentive for outages.

I don't use GAE, but in the comments at the original article there were people talking about huge amounts of instances getting spun up.

Does the 10% cover that in all cases? (Did they maybe roll-back any charges from those 6 hours, as well?)

Psychologically 10% free sounds more generous than one month free