You should be using 1ml of Flashlube per litre of LPG. On average that means setting it to 8 drops per minute at idle as a starting point. Then top it up to 400ml. When you next fill up with LPG make a note of how much you've put in, and then look to see how much Flashlube you've used. Adjust as required so it's using 1ml per litre.
So you should expect 500ml to last you 500 litres of LPG. On my 3.0 litre Subaru, I get around 300 miles to a tank. My tank holds 65 litres, so 65ml per tank means about 2500 miles per 500ml of Flashlube (very roughly!). I tend to buy it in the 5 litre bottles and decant into 1 litre bottles for easier filling. My wife's Honda CRV uses it too.
Instructions are on the Flashlube site by the way: http://www.flashlube.com/images/stories ... kit-s2.pdf