Hi Dom, That should be about right for you yes, 8.4 is the best size on the 121, followed by 7.7 and a 7.0 will also work on it.
Medium tuttle is something between normal tuttle and deep tuttle, some of the fin manufacturers use a tuttle head size which is slightly bigger than standard, but not as big as the Formula fins. So a normal deep tuttle fin will fit in there, but not the formula style very large deep tuttle bases.