Not how I see it.
I've not gone through it with a fine tooth comb but I reckon it's like this:
First - look at the league standings after ppg
Second - look at how well teams did, at the end of the season, for the last 3 seasons and compare this with how ppg, at the same stage as we are now, would have predicted they would have done by the end of the season. Use the comparison to arrive at the "margin of error" figure and adjust the ppg figure accordingly.
If a team, on average, over the last three seasons, had a better ppg in, say, the last 9 games of those seasons than the previous games in the season they would get a plus adjustment to the current season's ppg prediction. If they had a worse ppg they would get a minus adjustment.
This assumes that if a team did better/worse at the finish of the last 3 seasons they would do the same this season.
In my opinion it's total BS.
In reality it's "add the number you first thought of and if Tranmere avoid relegation - go with it"