If pubkey appear "under zero" after shifting it is not a matter you any way will found key.
Don`t use this method yet.
For test range is 80000000000000000000:ffffffffffffffffffff
I generate random key very close to begin range.
0x9000000f000000300001
without shifting key was found. with shifting - no. I don`t understand why..
I don't understand how it's helpful since you must know the private key in advance. Example, for puzzle 115, you have to know it's private key before shifting it down, but if you already have the private key, why shift down?