If you're not using the rich text editor, you can use JS:
%%MERCIFUL%%% Merciful/<script>document.write(100 - %%MERCIFUL%%);</script>% Ruthless
Or, make a separate variable for ruthless and update it in a link script:
%RUTHLESS := 100 - %MERCIFUL
And:
%%MERCIFUL%%% Merciful/%%RUTHLESS%%% Ruthless
Alternatively, just use a page script instead:
$PAGETEXT := $PAGETEXT + %MERCIFUL + "% Merciful/" + (100 - %MERCIFUL) + "% Ruthless"