I don't have an answer for you, just some guidance if you are interested. laying anything over about 7 could get VERY expensive, and you strike rate needs to be very high to offset any bet that goes against you. Don't for one minute assume that because a horse is at odds of 10 or more that it has little chance of winning. Horses over 10 come in all the time.
Not trying to teach you, just something that may help save your bank.