That method is at least extendable to other bases. For me, I probably would've converted to binary in my head and from there to octal -> 3 binary digits in each octal digit. Let's try: 64 + 16 + 4 + 1 -> 1010101 -> ..1 010 101 -> 125. Totally not extensible, useful only between binary, octal and hexadecimal.