Unfortunately backing those over the $5 mark severely reduces the strike rate and thereby increases the need for higher dividends.

I don't believe that because the SR is 20% $5 is necessarily value for any given top rater.

You must consider the other contenders.

ie. if there are 6 horses rated above say 95 would the top rater have a 20% chance of winning, assuming there are more than 6 horses in the race.