Hi again Dr Ron!
Averaging the performance_rating of a human athlete, or that of a horse IN MY OPINION is simply wrong.
Why would you include a run where the horse may have been off colour, or the connections did not try, or it was an unsuitable race?
I would say, select the best performance in the not too distant past, even go back to the previous prep.
This will tell you what the horse is capable of doing, and that is what we are after.
Good luck