Thanks. This makes the most sense to me. I am usually determining what my final volume should be by using preboil vol, preboil gravity and postboil gravity. This is obviously not accurate for me and I probably have incorrect approximations in beersmith for estimating volumes.
I think I need to work on calibrating a stick to put in the kettle for volumes...
The boiloff / losses entered in the volumes tab are what got me! I had entered some numbers that weren't always constant and, depending on the beer/brewday, would throw off my expected SG/volume. It took a bit of playing with the numbers (and reading about efficiency calcs) to dial it in.
Now I account for mash efficiency and evaporation in the software, but trub losses manually. I increased my batch size to 6 gallons post-boil, not including trub/hop losses.
YMMV - I adopted this procedure because of my system's quirks: evaporation rate changes radically with the season and my losses to trub are different based on style.
For getting your kettle volumes: I use a tape measure to find the height of liquid in the kettle, then plug the height into a spreadsheet to get volume.