That's pretty much the stage I got to with my dutching. I had these formulas but needed more work I reckon to make them useful....but if you do have a method for picking which runners to omit from calculations then I'm interested to hear more and maybe help with some testing

One thing is for sure crazybadger, if you don't have a sound selection strategy, there is no staking method that will help you. Believe it or not, Betfair does occasionally have a back market of below 100%. However, the calculations still come out as needing to spend thousands just to make maybe $20. So I think the best strategy is to use dutch betting on races where you don't want to back the first couple of favourites. The other strategy is to reframe the market with your own odds and only bet value horses.
The formulae I provided are for the most basic dutch betting methods. I personally have moved onto more complex calculations these days with savers, individual bet amounts for each horse etc. For that, there is no distinct formula to calculate bet amounts, I just had to create a program to do it. Having a sound selection strategy is still crucial though. My personal findings on dutch betting are that you do your form study as per normal until you can narrow down the field to 5 chances. From there you back the 2 most likely to win and have a saver bet on the other 3. That way you can cover quite a few runners and not bet too much at the same time. Also avoid odds-on horses, they will blow out your bet amounts and have an annoying habit of not always winning.