Max Block Dex Needed:
(75*2*99)/(36+X)+15 = dex needed rounded up (99 is your level, and 36 is the chance of block HS adds)

So, just put the chance of block your shield have in the X, and you are done.

For example, for a 60 block chance sacred target spirit = (75*2*99)/(36+60)+15= 14850/96 +15 = 154.685+15 = 169.6875 -> rounded up = 170 dex