How to adjust/vary a negative voltage
ATX power supplies have a massive current capability on the +12V bus. You could make an inverting switching converter.
Or, for less current and -1.25 to -12V adjustability at a few hundred mA, use an LM337 as jonk suggests:

Typical -12V bus current ratings on a 300W supply are 300-800mA. With a big heat sink you can use all that capability down to the lowest output voltage.