You know block is based on the following variables:
Character Level (clvl)
Item Block % (ib)
You also have your desired block value (db), which is equal to 75%.
Your givens are the ib and the db, as well as an assumed clvl. So, with the two basic assumptions that the required d will go up as your clvl increases, and if we assume your clvl is at 80, you can start working a formula together.
Or, you can look up the information, as Frank has suggested.