Boil off % can be a trap. As the boil off volume calculated from a percentage depends upon the pre-boil volume ie: 13% of 32L = 4.16L Using a percentage any variation in pre-boil volume will result in a variation in boil-off volume, when in fact boil-off should vary very little due to changes in pre-boil volume. Largely kettle surface area and applied heat determines boil off rate. All things being around average on your brewday, same kettle dimensions, similar boil rate, ambient temp and air movement around and over the kettle your boil off rate should be roughly the same volume each time. Set the boiloff in Beersmith as a volume from your records not a percentage. If I make a single batch I lose 8L over 90 min (5.4L PH) to boil off just the same as if I do a double batch. Beersmith would calculate a difference of almost double the boil-off volume for the double batch. Also just set all of your deadspaces and volume losses to zero. Set the batch volume to allow for these and you should find the calculated values more acurate.
And by the way I generally gain around 6 to 8 gravity points over a 90 min boil (not what Beersmith calculates by the way), mash eff is over 90% and brewhouse around 85%. I take the pre-boil gravity which is usually above what Beersmith predicts and know the end of boil will be around 6 points higher on my system.
Hope this helps you get to the bottom of the disparity.
Screwy
