This is an interesting question valueman and I would like to suggest one approach.
You handicap your horse at 9/2 but let me change it to 5/1 to be a little conservative and for illustrative purposes. Now the horse race can be compared to a dice throw with the chance of your horse winning being the chance of throwing, say, a six.
A prime concern now is the total loss of bankroll before a six is thrown (or a horse of this calibre wins). Now in one trial the chance of a non 6 is 5/6 or 0.833. The chance of a non 6 in two trials is 0.833 X 0.833. The chance of a 6 not being thrown in n trials is 0.833 to the power n.
This equation, 0.833 to the power n, allows us to select the level of risk to total bankroll. If we use the 5% level meaning the chance of a 6 not being thrown (or the horse not winning) being less than 5% then n is 17. This analysis suggests using 1/17 or almost 5.9% of bankroll.
The chance of losing 17 consecutive bets of this quality (0.833 to the power 17) and hence the entire bankroll is 4.5%.
Since your bet is 4.3% of bankroll you could theoretically make 23 bets of this size and your risk of total loss i.e. 23 consecutive losses(0.833 to the power 23) is 1.5%. This is indeed quite conservative and I suspect that you have carried out an analysis similar to that above.
Good luck!