The formula is basically a take on Ohm's law, V=IR (or in this case, R=V/I). Resistance is delP/CO, which gives you units in mmHg*min/L, which aren't the standard units of measurement. The multiplication by 80 is there to convert that number to dyn*sec/cm^5, which is more standard. More rarely you can see multiplication by 8, which converts the number to MPa*s/m^3.