This is the latest idea.

I wish to make $150 per week,betting win only or possibly each/way.And I wish to make a minimum of 15% om my investment.
So to satisfy these two criteria,I need to invest $1000 per week.
Now what is known as the "comfort factor" for me is a bet of $50.The largest individual bet that I can feel comfortable with.So to make the tally of $1000 per week,with maximum bets of $50,I need to have at least 20 bets per week,more likely about 30.
I would suggest that there are about 30 meetings available every week.So I will have one bet for every meeting available on the day.
To find which races I will attempt to beat,I add up all the API numbers,then divide by the number of runners.The highest figured races will be my betting races.
I will try rating a price for each winning chance and wager according to my rated price.But only if the available price satisfies this formula:

Needed price($)=(rated price)^2 x 0.15 plus the (rated price).