Sorry if this has already been suggested, I could not fin it.
As a third alternative to show the number of lemmings already saved compared to the required number, why not use a "saved/required" syntax in the skill panel ?
So for instance in Just Dig if you have already saved 3 lemmings, the panel would show "3/10".
The negative saved count in vanilla NL is nice, but still kind of weird to use a negative number. And the alternative offered in CE forces players to mouse over the saved count to see the required count. With "saved/required" you see all infos at once.