I got 6 right. For another 2 incorrect estimates an edge of my estimated range was very close to the correct answer. I think the key is not to try giving too narrow estimates and think in logarithmic scale. If it was possible I tried to decompose a problem to smaller pieces and calculate my estimate from information known to me (i.e. radius/circumference of the Earth for geographic questions) and make a range out of it by adding/taking couple of zeroes from my original estimate.
I use Mailspring (has both MacOS and Windows version).