I’m new here and I may be giving you wrong information, so if I am I hope that someone will point it out to me…but I think you would just need to put “/%postname%” and leave out the .html extension. I don’t believe that you can have (and I know you don’t need) a .html extension on post pages.
Again, I might be wrong, but I think that will work for you.
BTW, is there a reason why you would need to use the .html extension?