e.g. if i want to export a jpeg at 80%, how do i do that in Sketch.app? it appears to export at 96% by default.
there is a much more simple way of doing this. when you hit export and the file prompt shows up, click options, and you are presented with options including quality
after some investigation, i discovered you are able to do this via os x's command line
to lower the quality:
$ defaults write com.bohemiancoding.sketch3 JPGQuality -string "0.6"
you can reset it to the default of 0.9:
$ defaults write com.bohemiancoding.sketch3 JPGQuality -string "0.9"
i also learned that a variety of things can affect JPEG quality, so the percentage i set in sketch doesn't line up exactly with the quality reported by imagemagick's identify tool.
you can see Sketch's configurations by running this command:
$ defaults read com.bohemiancoding.sketch3