how do you figure out............how many of the 400 games at legend level before your average only goes down.....you have played if your score history does not go back to the day you turned legend?
There is no way of finding what your ranked games were going back THAT FAR.
Just start your count with a good guess as to what your ranked games are now I suppose.
I noted down how many ranked games i had when i turned Legend.
You could do a little math and make an educated guess if you know when you turned Legend. Go as far back as you can in your history and count the RRs as a Legend. Come up with a reasonable average of RRs played over that period and then apply it to the known date.
Unfortunately...with the advent of TL tier, I am told by those who know that you now need 500 rds before saturation occurs (or so they say). I found this out the hard way when I returned and my avg jumped back UP.