The mix for a scratch coat would be 4 to 1,,, plastering sand and cement. You would put a measured amount of waterproofer/plasticiser into the water. The thickness for the coat would be around 7 to 8mm,,, ish,, certainly no thinner,, but not much thicker either, otherwise the coat will be too heavy and slump/split. Consistency?? A fluffy, spreadable mix, with plenty of air in it.