Bios is my preferred method. IIRC, with the Asus bios, press f7 to enter advanced settings, find the hardware monitor tab (I think that's what it's called), where you should be able to find settings for all of the fans plugged into the motherboard. From there, you can either assign each fan plug one of three pre-made settings, or go full manual, typing in your desired fan speeds across your desired temperature range. Optimally, you should get each fan to spin fast enough to feel it moving lots of air, but quiet enough that it isn't annoying while you're using your computer. This applies not only to the case fans, but the CPU cooler fans and GPU fans as well. This process is generally referred to as tuning fan curves.