And the main feature of sha256 hex pvt key that makes it secure in the first place is, once you change even one letter, the whole underlying (input) string changes! This works both way, if the output changes (the sha256 32 byte) consequently this means that whatever comes out as a ripemd aaand the resulting address will change significantly. This means that you can have trillions and trillions of addresses starting with 13zb...... Spread around the entire 256 bit range leaving you with the fact that none of them is the exact address you're looking for. Someone then might ask then why would we search by prefix instead of addresses?! The answer is simple: it's way faster to look for part of the address. That's the only advantage. But It doesn't say anything about determining or even predicting the range.