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Ed
I’ve been thinking about D=pt/(p+t).  It’s dimensional rubbish, ‘adding apples and shoelaces’, ‘best guess’, ‘rule of thumb’, or ‘an approximation’ at most.  It only works exactly if t=0min when you would set the MPP to your DR position, which is also your last MPP, and if t=infinite minutes when you would set your MPP to your LOP, because your DR error would also have become infinite.  All it’s saying is that the longer your time between LOP and previous MPP, your MPP should lean closer towards your LOP.  Also, for a given t, the greater the distance between LOP and DR Position, the closer your MPP should lean towards your LOP.  I think I might question this second revelation.  It might just be an undesirable side effect of applying such a simple formula. However, using the approximation D=pt/(p+t) is a simple way of introducing MPPs into the student's work pattern. 
In the real world, most RAF Groups published a listed of expected errors for each type of fixing, a list of expected error rates for each DR method, which you multiplied by your time since your previous MPP, and you produced an MPP by dividing the perpendicular from your LOP to your DR position in that ratio. You had use the approved method, because all logs and charts were handed in after flight for peer assessment. 
Incidentally, the night of 16/17 May is the 79th aniversary of Operation Chastise, the Dams Raid, and also less than 24 hours after the full Moon if you were thinking of taking a nocturnal ramble. DaveP