Forecasting potential race performance with Polar Running Index

Polar is for me a very mysterious company... they "invented" the HR monitors, their units are packed of very useful functions... but then they do not make any effort to explain how to use in practice the functions made available...
The most mysterious of all is the Polar Running Index, about which I already wrote a couple of times (see HERE and HERE)

Now I try to use the Index as "forward looking tool" to forecast my incoming marathon time....

I own the Polar Rs800 since December 2006 and the trend of the P.R.I. is

Because I wanted to see better how to use Garmin and because the Polar food pod was broken for a while, I miss several data in the most recent period...

Anyway it seems that the past figures give a good idea of how my performance could be.
78-80 : around 2h42' (but that day in Boston the weather was tragic...)
80-82: below 2h40'
85: more around 2h35'

obviously these are my personal data and other people might have different curves.
Also the Index does not keep into consideration the specific training you could have for a specific distance (5000m o Marathon) but only a general indication of aerobic fitness
So I can bet on myself for Sunday that the time probably it can be something around/above 2h40', but then need to be factored by the hilly course and weather conditions (humid...).
I would be very happy to go below 2h42', qualify for Fukuoka and save somehow my legs for the spring marathon

Ok, now all is said and let's see what happens on Sunday