Because guaging render in buckets is as near as you can get to batched material, that is to say every mix being the same strength and colour and so on,
For plain render i would use 4:1 scratch (4 buckets washed sand 1 bucket cement) 1 cup of waterproofer every bucket of water
top coat, 6:1 (6 buckets washed sand 1 bucket cement) 1/2 cup of feb every bucket water, make sure the buckets contain the same amount of material level to the top.