Well, that formula might be abit confusing if your trying to calculate the dex required to maintain max block.
A similar variant, which will help you, is:
Dex for 75% blocking = 15 + (150 * clvl / Blocking) rounded up
Blocking = A total of the Blocking on all of your items. (E.g: Guardian Angel, Twitchthroe, Any Shield, Holy Shield)
I'd work it out for you, but I don't know what shield you made your exile in (or whether your wearing a GA, etc)
Best of luck.