The obvious solution is to offer multiple splat rulers and have a plain one as default
Consider also that NeoLemmix has rewind capabilities. Suppose a lemming falls from a window and splats. This usually happens immediately at the beginning of the level, and so the player thinks "aha - need to fix that". Usually this is part of a level's puzzle anyway. In this case, the window splat marker is useless
Why not have both? The splat ruler could be changed to remove the climber/window behaviours, and the skill shadow system would cover those cases by itself.
The hatch drop is 100 % local and doesn't need a mobile ruler. You can stick the splat marker for a hatch into the hatch's hover-during-clear-physics information
The solution to the problem lies in these four comments, IMO.
For hatches, it's nearly always obvious when it's splat distance. And if it isn't,
let the game start playing and find out very quickly! For the die-hard "picture puzzlers" (i.e. those who absolutely don't want to start playing until they have all the information they want/need), we can add the info to CPM as Simon has suggested (and, we can add it as a non-CPM helper image at the very start, along with pre-assigned icons, etc).
For climber fall distance, I like Proxima's idea of a separate ruler. A shift key modifier for the regular ruler should be sufficient, or we can allow its own hotkey, of course.
Walker fall distance can use the no frills splat ruler without additional markers - we just need to decide exactly how big to make it. Personally, I like mobius's "only show fall height" ruler (with no markers) - the top side of the terrain piece at each end of the ruler must meet the ruler for the fall to be safe, simple*.
Meanwhile, Tan's idea of showing fall distance when hovering over a faller is brilliant, and very easy to achieve alongside everything else (as opposed to instead of it) - I'd suggest simply showing the safe fall distance, no need for extra markers/information. If the shadow meets terrain, we know the fall will be safe.
*I've prepared a version of the ruler which fulfils this. I'll be making this one the default for SuperLemmix, but obviously anyone is free to use it in their copy of NeoLemmix. For reference, the fall on the left image is safe, the fall on the right image is fatal:
The first attachment is low-res, the second is hi-res. Place them into
gfx/helpers and
gfx/helpers-hr.