Higher than 67% seems insane to me. Those sorts of numbers are basically never seen in democratic elections.
Very dominant parties like Japan's LDP, the UK Conservatives, Sweden's Democrats typically get 40%-50%, occasionally up to 55%
But I suspect the best analogue would be newly democratised countries like South Korea, Bulgaria, Taiwan or Mexico where the former undemocratic rulers become a powerful democratic party.
If that's the case, why not test it in a non-gerrymandered election?