How do I calculate mash efficiency with BeerSmith2 when doing a partigyle?

Since you already have your wort, and know the volume and gravity, you don't need an accurate grain bill for that part, but just need to get the software to register 3.6 gallons of 1.034 wort. You can do it like this:

1. create a new recipe and set your desired batch size.
2. add 1 pound of base malt, e.g. pils malt.
3. Click "gravity" and enter 1.034 as the target gravity - the software will now increase the amount of grain you need.
4. Click "scale" and enter 3.6 gallons - the amount of grain will be scaled to give 3.6 gallons at 1.034. (Since we already set the batch size in step 1, the recipe is already to scale so this part can be skipped.)
5. If you want you can also approximate for color. Add 1 oz of black malt/roasted barley, then choose "color" and enter an estimate of your wort's color. Beersmith will scale the amount of the dark grain to approximate the color. It's usually a small amount compared to the base malt, so gravity remains unchanged.
6. To hit the SG you'll need to add top-up water to compensate for the boil-off - since the gravity units were computed using the post-boil target.

With the recipe emulating what you now have going into the kettle, you can then start to add adjuncts and hops to the recipe and and the additional gravity and IBUs will be computed correctly.

(PS: I'm using beersmith 1 - you can do the same with beersmith 2 by using the sliders for gravity and color) and presumably any other software that scales ingredients to give control over gravity, batch size and color.

The way I did it was to duplicate the recipe, adjust the volumes until the sparge volume on the first is 0 and the mash volume on the second is close to 0, then fiddle with the efficiency settings on the two until the estimated OG matched my measured OG. I think it ends up around 58%/25% efficiency for the first/second batches. Example recipe link here.

Obviously not the easiest way to do things, but I couldn't come up with a better way to do it.

Edit: also check out the Beersmith forums. I just did a search and there are several threads that talk about parti-gyle brewing.