smart risk tolerant money to be made, page-12

Sorry trouser but you have got it wrong.
The different probabilities are weighted and so can be added together to give a mean expected outcome. The p50 figure is not exclusive of the p90 figure - rather there is a 50% chance that the field is at least the size of the p50 and so you can weight the numbers then add them together to get a mean expected outcome.
Dividing the number by 1.6 is not necessary nor is again dividing it by 1.3. You forget that the standard industry multiple of 0.2 has been given the 1.6 multiplier on the basis of the assessed geological likelihood of success. 0.2 is just the industry standard starting point and to remove it to convienently get the current SP seems just an attempt to make the figures 'fit' (thus justify) the current SP.