If only it were that simple! There's no-such-thing as 'perfect motion blur', it's all a matter of taste.
You can do things like open the virtual shutter to full-open (so that not a single virtual-photon reflected off the virtual-motion will be missed by the virtual-sensor), which makes the motion recorded during one frame continue directly (with no time 'gap') into the next frame. This can help make things seem 'smoother' for sure but it's still relying hugely on persistence-of-vision.
I'm super surprised (and a bit incredulous tbh!) to hear of a perceivable difference between 120 and 240fps! need to go double-blind on that stuff to be sure I reckon!