All grain wort volume- guessing game?

Greetings,

I just brewed my first all grain batch last weekend and was pretty close to the desired result. The OG was a bit off, but I think that was due to under-mashing. I used a 7 gallon wort volume and ended up with about 5.5 gallons of wort, which was the aim. However, in my research, the guidelines for this are pretty loose. What formula or advice can be offered for determining the wort volume to end up with a specific gravity or desired volume. Everything I read says that you'll lose 1-2 gallons in the boil based on efficiency. It seems like a guessing game to me. I would have to imagine that someone has pinned this down. In other words, how do you calculate the boil volume to get a 5 or 5.5 gallons to put in your fermenter? Any advice?

Thanks,

Brad Wyant