i actually do things a bit different,

i save out of photoshop an upload version, which is a max quality .jpeg, 

then i open up a freebie image editor, bring that image in, and then save to my rendo upload folder, sometimes at 100% quality setting (still .jpeg) sometimes that drops the file size enough, otherwise i just drop the quality setting to 98% and so on, until it is the right size.

i don't tend to lose any image quality that way.

photoshop seems to save out at a bigger file size than nessersary, so the "save for web" option would lower the quality settings more dramatically to get it to a certain file size, whereas if you use a freebie editor after photoshop, you don't need to drop the quality settings as much to get the right file size.