This post is awesome. I know I'm a bit late but generic quality values have a big catch. I tested retina images for one site and used I think 40% quality for retina too. The problem was that there were a few images that showed a big gradient in the sky. The quality of that gradient was horrible. The solution I came up with (because it always has to be fast) is that i just increase the dimension by 1.5x and set the quality to about 65%. With my knowledge now I'd have a different approach. The jpeg library used by imagemagick has the option "jpeg:extent" with which you can define a file size limit. If you get a high detail jpeg, it'll try multiple lower qualities to get within your restriction. The normal quality option becomes a max limit in that sense.