Efficiency is most relevant for all grain and partial mash brewers. It indicates how efficiently sugars are extracted from your mash. This is best based on previous batch experience. If data is unavailable then home-brewers can expect efficiency between 70 and 80% whereas commercial brewers can attain efficiency as high as 90%. Starting with 75% is going to get you close you will adjust this number as you gain experience. When you take your OG reading the efficiency can be more accurately calculated using the Analysis Tab.