You're on the right track.
We used to shoe Standardbreds several times in a DAY in order to get them
properly balanced. Pain in the butt, but worthwhile.
We use World Racing Plates for the same reason you're using Polo plates.
Don't bet your booties that hind angles are necessarily higher than fronts. I
have Australians that are successfully shoeing their trotters 54 in front 50
behind.
ti