When I’m exporting JPEGs in Fireworks I can set the quality in percent and I notice a clear difference in qualtiy between 85% and 86%. So usually the optimal quality/size setting is at 86% for medium sized images (in terms of dimensions). At large images you have to determine according to their purpose. Unless you have high qulity images for a photo gallery you should keep your images below 50KB if possible. Now I guess an image of 900px width can’t be keept at this size without losing a lot of quality so there do whatever you feel looks reasonable. If the image looks the same to you at 250KB and 150KB then use 150KB by all means. Reduce the quality of the image to like 10 or 20% and see how this looks like. Then you’ll probably notice a difference.
In the end you should do whatever you feel looks acceptable while keeping the file size as low as possible (but as high as necessary).
Usually, if photos were taken with a high quality camera the visual impact on these images if saved for web with lower quality isn’t as drastic as a photo that has been taken with a low quality camera in the first place. The above image looks perfect to me, probably also due to being of high quality in the first place. For the web this looks optimal.