Isn't the maximum value of private keys based on the p of secp256k1, which is  2²⁵⁶ - 2³² - 2⁹ - 2⁸ - 2⁷ - 2⁶ - 2⁴ - 1 ?  It's close to  2²⁵⁶, but not exact.  Not to be pedantic, just want to make sure my understanding is correct.