Yup I didn't allow for the floor joist sticking down
8.5 may be to code they will be awfull to walk down. There is one other cheat that you could look at to gain a few inches, if there is a wall directly above that floor joist. We have raised that joist bottom to 3 1/2" below the floor. Yes thru the floor into the wall above add a 2x4 to side of it to hold up the floor. If that would work, you would have better numbers to play with maybe.