720 * 32/27 = 853,3333333333333... 854 is the nearest number, but it’s only mod2. 852 is mod4. If you have anything in the processing chain that requires mod4 (e.g. AviSynth) pick 852, otherwise 854.

Mod4 efficency will be lower than mod16. But the penalty is usually too small to worry about.

I should also mention that mod4 is about as low as you want to go in terms of playback compatibility. mod2 just creates pain with too many combinations of video card + driver.