At Brewer's Friend you can also setup a recipe and adjust the efficiency to see the effect on OG. Or, you can 'brew' the recipe, then enter brew day logs and it will compute four types of efficiency: conversion, pre-boil, kettle, and brewhouse. We have robust definitions for 'batch size', and 'efficiency', since there is too much conflicting stuff floating around out there.
Here is a post by Kaiser at that provides complete details and a nice diagram:
http://www.brewersfriend.com/2012/11/30/making-sense-of-efficiency-definitions/