Duritz,

You seem to be looking for a formula to guarantee you'll never lose more than a certain amount of your bank. It's a crock and no such formulas exists except beating the bookie over the long term. OK, so you want to make sure you can only ever lose a 1/3rd. of your bank? When you do, what then? Walk away from punting?

Most punters who just enjoy a punt on Saturdays, allow so much per week to punt with and divide it up for the amount of bets they have [don't knock them, it's cheap entertainment if the betting allowance is within their budget].

If however, the goal is to beat the races and using a betting bank, forget all the maths formulas and runs of outs garbage and leave it in the hands of the mathematicians here [there is at least one in this thread I know of ..Stebbo], or would be mathematicians here who enjoy that stuff. Just bet 2% of your bank each bet. It's a fairly acceptable and common percentage among Pro's and serious punters for very good reasons. $20 bet for $1000 bank and $2 bet for $100 allows for an awful lot of outs. Just round down to the nearest $1 or $5, depending on your bank size.

You'll either [very] slowly get rich ....or poor :-(