Each type of grain has a different potential gravity. It will not be accurate if you pick a number from thin air and apply it to the entire recipe. That said, knowing your efficiency isn't really that important. After a few batches, you'll learn your system and what you need to do to get consistent results. At that point you'll know roughly what your efficiency is based on how close you are to your target OG.
If you're OCD about numbers and absolutely must know, there are programs on the web that you can punch your recipe into and calculate your actual efficiency.
I aim to misbehave.