Correct, the flip up lens offers no enlargement, it just allows your eyes to focus on something really close to your face. The assembly gets called a "flip-up magnifier" but it doesn't actually magnify anything.
IIRC, the -1.5 is the correct value for "standard" eyesight. If your optometrist has told you that your vision is 20/20 or better at both near and far distances OR if your eyesight is corrected to 20/20 with glasses that you wear while looking through the flip-up magnifier, then the -1.5 diopter is probably the right one for you. But really, you just use whichever diopter makes it easiest for your eyes to focus on the ground glass.
An easy way to experiment is to take your camera to a drug store. Almost all drug stores have a display of "reading glasses" which are simple lenses ground with solely spherical correction. They'll range from -0.5 or so all the way down to -4 diopter. Leave the flip-up magnifier out of the way and see what power of reading glasses make it easiest for you to focus on the ground glass with no eyestrain when you have your eye right at the top of the WLF chimney (right were the flip-up magnifier would otherwise be). Whatever diopter power of glasses you feel are the easiest to focus through is the same (within 0.5-sh of a diopter) as the lens you'd do best with in the flip-up.