It depends on the dog but from what I can work out from researching on the internet a dogs diet should be about 80% meat. I can't find anything that actually gives an amount of raw food to feed for different breeds.

Luckily my dog won't eat more than he needs so we give him 250g of raw meat a day and leave his biscuits down all day. Like you I want him to go on a totally natural diet and will probably give him 500g of raw meat a day suplimented with vegetables etc. I think it will just be a case of keeping an eye on him. If he loses some weight give a little extra and if he puts some weight on give a little less.