I think it helps if you explain to you're client what the rules are , I once did a job taking some people from London to Cardiff and explained the weather on route meant it was very unlikely we would be able to get there but as the weather in London was fine and they where paying we set of .
I explained that if the cloud base was less than 500 foot I would be breaking the rules as it's impossible to do the route solely over empty fields and set the rad alt to 500 foot , then about Swindon the rad alt went off and little whisps of cloud where passing us so I simply said that's it that's as far as I can go .
After landing back in London they never questioned my weather decisions again
CBS