Likely not the CPU fan or OPT headers. "CPU Fan" is a standard recommendation across multiple cooling types since you will get a boot error if there is no signal detected on CPU fan. The problem with doing this with a standalone D5 or DDC pump is you will be tied to CPU temperature. You definitely do not need/want your pump to react to CPU temperature and you get set the BIOS to ignore the empty header. On the Formula you also have an AIO or W_Pump header that can do the same thing, except of course it also is tied only to CPU temperature on most recent chipset series. Someone told me they finally corrected this on the Z390 boards, but I can't confirm. If you have other control options for W_Pump besides CPU temp, I would use that. The ideal control variable is going to be you water in/out or T-sensor 10K thermistor connector on the lower right part of the motherboard. That can go to any sensor you are using in your loop and give you a pump change based on coolant temperature. You don't really need fine, granular pump tuning and fixed speed works fine for most people. However, it really complex, large, or multi-GPU loops, you may need the extra pressure to get things around but that is a bit noisy for quiet work. If it is a miss on the AIO/W_Pump header, your best bet is any CHA or HAMP header that is convenient.