How do I calculate for a Missed Pre-Boil Gravity?

Of all things. I forgot to get a Pre-boil gravity reading and I really want to know how efficient my mash was.  It there a calculator out their that can do this for me?

#'s that I have to work with...

Sparge Final Run Off Gravity = 1.030

Pre-boil volume = 12.6 Gallons

Post boil volume = 11.25 chilled to 68 Deg F.

Open Gravity = 1.059

Dan

Since you know what your pre and post boil volumes were, you can use the boil off tool to get your pre boil gravity.

Thanks for the tip BrewFun. It worked great and it calculated a Pre-boil gravity of  1.051 so my mash Eff = 79.8%