I want to do 100%, but I want to reset my progress so I know with certainty whether or not I get a par on a level. How can I do this?

I've found out now - if you play it in private mode on your browser, then your progress is cleared each time you refresh the page.

If you use the standalone flash player, just load a .swf file then Right Click > Global Settings > Delete All...

Hopefully this helps for people that use the standalone flash player.