I thought you did know Ed. I was confused by one of your responces.
Just as mine is confusing. My example was based upon 102,5Y 92,5M being 52.5y and 42.5m higher than 50y,50m, 0c. This would make the inverse (-52.5y and -42.5m from 50y, 50m) 0y, 10m, 2.5c not the number I quoted.
There is a reason I sport a fried egg.