Interesting example XPT.

The SP prices reflect the 18 runner field size, so I wouild be looking at adding from 75% to 100% to the prices needed before calling any price an overlay and even if 100% was added there are still some among your runners there.

Working out a simple %+ guide for various common race sizes would be pretty simple. Even for a 6 horse race though, I would want some % over to represent profit on a runner's actual 'odds' shown after converting from SR.
At least we wouldn't be betting blind value using this method.