For the ones that I have done I just use two carb drops per bottle. This works well and is nice and clean. However, as time goes on I may just use the cheaper sugar option. For most this gave a nice amount of fizz apart from the cider that I tried. These was not enough in there, so next time I may do three.
Reading the posts above I didn't realise that I would need to put less in for English Bitter but thinking about it yep it is right. When I get back to the UK I will be doing one and do not want it too fizzy.