With some further testing:
If the rope slide is flat - the lemming appears 2 slabs too low
If the rope slide is 22.5 degrees downwards - the lemming appears about 1 to 1.5 slabs too low
If the rope slide is 22.5 degrees upwards - the lemming appears about 1 to 1.5 slabs too low
If the rope slide is 45 degrees downwards - the lemming appears correctly
If the rope slide is 45 degrees upwards - the lemming appears about half a slab too low
(All distances in this post are still estimates.)
Note that the first case, a flat rope slide, never appears in any offiical level, nor in any custom level so far outside of one made specifically for testing. One could argue that instead, the rope slide itself should be placed two slabs lower in this case - that would make more sense visually anyway, IMO.
