Ok, I know how to save for web and I also know about the optimize to file size option too.
The problem I am having is that Save for Web is missing the specified target size. It always misses the file size, by a little bit. On images that have a nice smooth BG or little BG details the margin of error is small enough not to make a difference.
The trouble comes in when saving a file that has a lot of detail in the BG. A couple of recent examples are a a squirrel spread out flat on a tree trunk ( a lot of detail in the bark patterns which fill the frame) and a macro of a fungus (lots of detail in the gills which fill the frame).
I used the optimize to file size option on the fungus and set it to optimize to 100kb, the actual size on disk was over 180kb. For the squirrel, I set the file size to 100kb and the actual size on disk was over 160kb.
I've heard others mention that save for web has a tendency to underestimate the final size but never really noticed until I tried it on images with highly detailed BGs.
If anyone doubts it, note the size that save for web says it will produce, and then look at the file's actual size. Again this will be more noticed with an image with a LOT of frame filling details.