Maxis says for the instance piece of a new (non-overwrite) resource file to use:
The 64-bit FNV-1 hash of your name and the tuning instance name, with the high bit set to 1.

I get the reason for making the instance very unique so as to never conflict with other modders' files, but how does one go about setting the high bit to 1 exactly?

If the first byte of the FNV64 is 7 or less, e.g. 475CD082C59B67C2, add 8 to the first hex digit, so this example the first digit should become 12 decimal, or "C" hex. The final FNV would then be C75CD082C59B67C2.

I have a tool that calculates the hashes and sets the high bit for convenience. It can be found here: http://fogity.boards.net/thread/4/modding-toolbox

Thanks guys, I knew it had to be something silly simple!