To create a successful staking plan, you need to know what you will be dutching....
If you are working on a 50% win strike rate you could increase by about 120% and decrease by about 80%. You need to accurately determine your strike rate though, I don't think you can pick your own horses. I think you would have to dutch the first 3 favourites or anything under $10 or something similar.

Without knowing a reasonably accurate and consistant strike rate, you cannot create a staking plan for it.... If you were to dutch the 3 longest horses in each race and use the figures I quoted, by the time you had a winner the bet would be so high that you would back the horses right in anyway.

Oh, and you must use fixed odds......