I remember playing round with a system such as that, looking for races where the fav is vulnerbale. I think I had the rule that the fav must be 3.50 or more, and then you would back as many horses as you could, usnig the second fav as a saver, ie if second fav was showing $4, back that and three others. I was only paper testing it, but it went really well, until one day it hit a horror run, where either the fav or a rank outsider got up for a large period. Might have to look at it again, and apply a divisor target betting plan and see how we go.