Adding rpm/100 to the MP cannot mean anything, except as a coincidence of numbers. The two parameters are different units.
A more accurate HP estimator is to take the
LOP fuel flow and multiply it by a certain factor (which I can't remember
). This should work for any petrol engine with a given compression ratio.
I fly a TB20 and one aspect of POH performance which has intrigued me is fuel economy. Socata's figures for best economy cruise are pessimistic by some 15-20%. It appears that they just lifted Lyco's power settings and derived the performance (range) from those. But the practical difference between a zero-fuel range of 1100nm and 1300+nm is huge, when doing real flights around Europe's often sparse avgas+customs airfield situation.
Since Socata did not change their POH since the late 1970s (to greatly simplify certification of the GT version in 2000) and since accurate fuel totalisers were not around in the early years (for GA) I can understand they probably never knew the real fuel flow rate.
OTOH Socata's runway performance appears spot on, suggesting they derived it from test flights.