Mad Gambler,

You can only work out 'true' odds in Casino games. If it was possible in horse racing, there would be no horse racing.

Here is a simple method for what you are looking for:
Take the top 5 or 6 horses PP and create an 80% market [forget odds, they are actually percentages]. Divide those runners into what % chance you think each horse has of winning and then convert the % to odds. They are the prices you are looking for. Above it's an overlay, below it's an underlay which naturally you don't except.

You'd be surprised at just how good you get at it after a bit of practice. If you can't do it or you think it's too much bother, except losing and just enjoy yourself and call punting entertainment.