There is only one camera that I'm aware of (Canon 1ds MII $8000) where you
can specify the actual compression ratio of a jpg image. In camera, rather,
you set the image size, and then the type (JPG, TIFF, RAW). This is how even
most high end cameras (like my D1x) works. The jpg image is saved at a
preset compression based on the output size specified. If you want to LOWER
that, then use software.

The ratio that you are attempting to determine is NOT something you can
mathmatically figure with a simple calculation as jpg size is based largely
on WHAT is in an image. A lot of Blue sky = smaller image. A lot of varied
colored detail = bigger image. One can NOT assume a compression ratio based
on the image size alone.

Shooting RAW has advantages over most other formats. RAW is a 16 bit format
(actually 12 for most) vs. the 8 bit jpg and TIFF formats. Some of the newer
DLSR's have a 16bit TIFF option.

The bottom line is that if your biggest concern in shooting a PnS is the
compression ratio of jpg, than you are likely not going to have to worry
much. There are MANY things that can impact the ability to shoot high
quality images with a PnS digital (as good as they are). So to worry about
jpg compression is rather pointless.

Shoot with the highest res possible, and if you are unhappy with your
images, then you will want a better tool.
Ron