Preboil gravity being calculated wrong on the BrewSheet

I've been having issues with varying amounts of wort lost to kettle trub, and it's effects to my "brewhouse efficiency" in BeerSmith. Tinkering with my equipment on one of my recipes in BeerSmith I changed the volume "Lost to Boil Trub and Chiller" to 20 gallons (for a 5 gallon batch, just to make sure BeerSmith is giving me good information); then I opened up the "BrewSheet" which is what I use when I'm brewing for determining volumes and my targets etc.

So check this out; the BrewSheet says "Sparge with 30.28 gal of 168 degree water, add water to achieve boil volume of 35.98 gal" then "Estimated pre-boil gravity is 1.051 SG with all grains/extracts added"... Mind you this is a 5 gallon recipe with about 16 lbs of grain. So I change the "Lost to Boil Trub and Chiller" number back to 0.5 gal, and then guess what? The preboil volume is now 8.71 gal and the pre-boil gravity is 1.050. The estimated preboil gravity actually went down 0.001 point. Craziness.

If I'm doing something wrong... please tell me what it is. Otherwise I realize now I can't trust the software to give me a good preboil gravity/preboil volume to be shooting for.

I can't speak to how BSmith handles such crazy curveballs, but I use the pre-boil and final OG numbers more than I do the efficiency %, as a measure of how I actually did against what I expected to do.  It's taken a while to learn my system (and to quit screwing up), but today I nailed both numbers on a porter. 

If you post an actual realistic recipe file, people could take a look for possible equipment settings that are messing up your numbers.

Hello,
I too have experienced this today, and i *think* i know what may be happening.
My case: 3 gallons pre-boil, .50 gal lost to trub for a total of 5 gallons at 1.056.  Est pre-boil gravity = 1.103
If I change my losses to .25 gal, keeping the 3 gallons pre-boil volume, 5 gallons batch and 1.056 OG, I get an est. pre-boil gravity of 1.098

What I think may be happening is that Beersmith may be working "backward" from the final batch volume to estimate the pre-boil gravity.  So that in my case it is saying that if I want 5 gallons batch with .50 gal loss, I better have a pre-boil gravity of 1.103.   I can get by a pre-boil gravity of 1.098 if not so much wort is lost in the trub, to achieve the same OG of 1.056.

It is a tricky way to calculate pre-boil gravity, and not sure if it is the common practice, as I am a novice, what do I know?  I hope someone more knowledgeable can enlightened us.
Cheers!

note: I think the correct pre-boil gravity is calculated when setting the trub loss to 0.0

Hi,
BeerSmith does calculate the boil gravity working backwards from the original gravity and boil volumes.  Also if you don't have the "Set Boil Volume Based on Equipment" it is likely the numbers won't add up since essentially you are overriding the internal calculations.

Brad