Question about computer power supply torroid?

We can't give you a formula, because we don't know the specs of the core of that torroid.

If you have something that can measure inductance, then you can keep adding or removing turns until you get to a desired inductance.

The next trick is the frequency. It looks like this is in a switchmode power supply (and it appears to be a transformer), so I guess the toroid core is OK for around 100kHz.

The next trick is determining when it will saturate. If you know the number of turns and the current passing through it then you can also calculate the saturation current for a different number of turns.

 

If you know the saturation current is 1A for 10 turns, it will be 0.1A for 100 turns. (turns times amps remains constant).