The one factor you're overlooking is your own "efficiency". Basically, the effectiveness of extracting fermentable sugars from your mash grains. Beersmith disables this variable for extract brews, but for PM and AG, it's key.
You can add exactly what beersmith calls for in DME to hit a target OG, but if you mashed your grains poorly, you've already fallen short.
You'll have to deal with the same thing AG brewers do, and that's determining your own brewshop efficiency and adjusting your recipes accordingly.