laying the field in horses

Anyone do or tried this with any success?

I've been laying the field with Gruss and trading out with an offset quite successfully,most races can be catered except v small fields.

Very low margins though indeed and you have to go in running and often close out as you'l always have a couple of selections maybe upto 3/4 that arent matched pre off.a gambled fav would be an example and i have a cancel for levels or take a loss facility for these outcomes.

Anyone tried any similair?