Having scales when mixing plaster is the most reliable method but if you don't have them you can gauge how much to add by looking out for islands of plaster forming on the surface of the water.

Always add the plaster to the water and only at a handful at a time, so not too fast. Don't mix the water and plaster until you have finished adding all the plaster, otherwise you won't be able to gauge how much you have out in and are also more likely to get lumps