I don't know why you want to know ratios for CPU-NB as you don't change a ratio number in bios but you change the frequency. At least on Asus boards, i don't know what board you've got?

For my Crosshair V Formula-Z i can set CPU-NB as low as 800MHz to as high as 6200MHz.(this is with default 200MHz Ht ref freq.) In other words from multiplier 4 to multi 31.

Hope this helps.