Originally Posted by
Homebrew01
Make note of the easiest gear that you ever use, on your toughest section of road (front chainring combined with rear cog). Perhaps it's a 34 front and 23 rear. So, your next cassette could have a 23 for largest cog, or more common 25 "just in case". Then you will have smaller steps between gears. Since you're in a super flat area, you won't be going downhill at 50 mph, so might be better of with a 12 small instead of 11.
+1 to what Homebrew said.
If you have a compact crankset and live in a mostly flat area, a 12-25 11speed or 12-23 10 speed cassette works really well(at least for me). It gives you a tightly spaced range of 12t-19t and a few slightly wider spaced cogs for short hills. If you get into some real climbs, you can drop down to the 34t chainring and have enough to climb larger hills.