Batch Size Calculation

As I understand it, batch size is the amount left in the kettle after the boil and would be expected to have the OG of the program calculation. 

What happens when I need to add, for example, 1 gallon of top off water.  I then add that to the program.  Will that then change the expected OG to reflect the gravity of the boil amount plus the top off water?  Will that reflect the right figure if the top off is added in the kettle as mixed with the trub?  Or will that be more accurate if the top off water is added after the wort is in the fermenter and leaving the trub behind?

The reason I ask is that I will have a gallon of trub loss and have that recorded in the equipment section. 

I hope I have been somewhat clear in what I mean.

I am not sure if I understand the question completely.

Are you leaving behind the hop sediment in the bottom of the boiling kettle and adding water or are you adding water after fermentation?  From my understanding, trub is the mush on the bottom of the fermenter after fermentation (i.e. the result of flocculation of the yeast).

One way to determine if BeerSmith is taking it into account is to exaggerate what is being left behind and what is being added.  The as the volume increases, I would expect the OSG to decrease based on the same grain bill since sugars are left behind and replaced with 1.000 gravity water.

Does this help at all?

I guess a test would be in order.