Put some water in your keggle, enough to cover the drain valve and then open it up and allow as much water to drain out as possible. Close the valve and pour out the remaining water into a bucket and measure what is left. Now, do you tip the keggle when you drain? If so, then repeat above and tip the keggle as you normally would. Now measure what is left. That number is your trub loss. Adjust your equipment profile with this loss
Since you are doing BIAB (I am assuming full volume), you don't worry about water to grain ratio. All your water will go in up front.
Next, look at your last recipe brewed and find the actual mash efficiency calculated by BeerSmith (not the estimated). Copy the recipe and rename the copy as "actual" or something which distinguishes it from your original. Open this file up, change the equipment profile to reflect your new wort losses and make adjustments to the "BH efficiency" until your estimated mash efficiency matches the mash efficiency that you actually attained when you brewed the recipe.
Now reopen your equipment profile and enter this BH efficiency into the mash profile. Save the file and use this for your next brew. Repeat this a couple of times and beersmith will get better at predicting what you will actually achieve.