I believe the "digits" above refers to decimal digits, not binary digits. RSA-240 has 240 decimal digits. A 1024-bit number should have 309 digits. So: 2^((

I believe the "digits" above refers to decimal digits, not binary digits. RSA-240 has 240 decimal digits. A 1024-bit number should have 309 digits. So: 2^((309 - 240) / 5.5) = 5978