You definitely want some preload on 50N springs otherwise youll be eating away at bump travel.
There is not a massive amount of free travel at the back on 60N springs.
Off the top of my head: you have apporox 220kg unsprung per side and damper have a gas pressure force of ~20kg. with a motion ratio of 0.56, thats a force of 395kg on the spring.
The rears have 80mm of free travel, so with 60N/mm you use up 375/(60/9.81) = 65mm, leaving 19mm/0.56 = 33mm of bump stop free wheel travel with zero preload.
With 50N/mm you use up 375/(50/9.81) = 73.5mm leaving only 6.5mm/0.56 = 11.5mm of bump stop free wheel travel. So id recommend about 12mm preload as a starting point.
There is a typo in the manual for the MP20. I am sure it should be 38mm on the back. 375/(30/9.81) = 122.6mm minus 38mm = 85mm. Its sitting on the bump rubbers a tad at this point.
Bare in mind with all of this that the bumpstops are about 40mm and quite progressive as well as the fancy damping, but the data above is for reference.