I know how to work the formulae for ratings points gain and loss but I can't seem to find how a players average rating is calculated
I thought it would be (highest +lowest) / 2 but it 'aint.

How about the sum of all the ratings of individual games in a given time period devided by the number of games?

Could be I suppose.
Interesting idea.
If i come across anyone who's not played a lot of games i'll test it for the 90 days average.
Trouble is of course such a player is likely to have played unrated games which wouldn't count.