Hi Woomf,
Depends a little on your intended sailing style on the ST104, and also your rider weight, but in general you can consider 36-38cm for light 6.8m, 34-36 for medium 6.8m, 34 for powered 6.8m and even a 32 if you are going for overpowered 6.8m speed in flatter water.
A 34cm is probably a good ST104 second fin option. In general.
Cheers ~ Ian