I now found a couple older posts about this. Bikeforums needs a better advanced search function!

I see that many singlespeed / fixed gear riders use about 60 to 80 gear inches. Assuming these are bikes with 170 mm cranks and 700C skinny wheels, that's a gain ratio of about 4.5 to 5.5

My Schwinn is set up at about 60 gear inches and a gain ratio of 4.5 with the middle (direct) gear, so that makes sense as a good single-speed gear. But I rarely find myself in the top gear, which at 80 gear inches and a ration of almost 6.0 is usually too hard on the flats, even with a tailwind. And the low gear, at 45 inches and a gain ratio of 3.3, is not quite low enough for the steeper hills.

Going to 8 speeds and a "305%) (1:3) ratio between top and bottom gears will be a big improvement.