You need to know you strike rate for top selection, top 2, top 3, top 4 and then work out the break even point so you can then work out the % profit you need per race. Don't bet every race as not every race will give you value.

For top 2 you might need a 300% overlay, for top 3 you might only need a 100% overlay and for top 4 you might need a 50% overlay.

It all depends on how good your ratings are.