Practise practise practise, thats the way to work out how much thinners you need.
Or find a paint mixing shop in yellow pages and ask them to make it up for you ready to spray. They may even be able to some in an aerosol as well for those days when you can't be bothered to get the compressor out. Whatever you do though, do not start spraying your bumpers until you've mastered the art. Get an old door from a scrappy to practise on until you can apply without leaving the dreaded orangepeel effect
Good luck