I would use my formula, but if you insist on being silly, at the least I would change the tests to stop being so damn ugly.
if self.USER_HEALTH <= -10:
...
elif self.USER_HEALTH >= 10:
...
else:
...
This does not assume, as your expression did, that the handicap level won't somehow escape ±10, and naturally clamps it while letting the normal range fall into its branch without that nasty (and at a casual glance, fucked-up) and partially-redundant test.