Here’s a more general ICE tree for inverting weightmaps: it handles weightmaps that are not in the 0-1 value.
When I first worked it out in my head I saw it like this:

But that’s just the same as using a Rescale:

Here’s a weightmap before and after being inverted by the above. In this example, the weightmap has a value range of -1 to 3.
Before:

After (inverted):

Looking at those, maybe I should do something like this:

Which gives this inverted weightmap: