My washback is calculated to include 400g for saline and 200g for my cup of tea and a sandwich. If I bring along a cake, my dry weight might go up 100g. No big deal.
That's pretty much what we do in our unit.
As Rerun said try to be consistent with dialysis - wear similar clothes (so you are weighing like with like) and take note of what you are taking in and you can adjust accordingly.
The other thing is - the only real issue is the fluid you take in. If I have a 200ml cup of tea(as I do) that's what counts. We also have sandwiches which I guestimate are around 150g, but if I come off 100g over again it's no big deal, specially if that's food weight rather than fluid.
If I have a second drink on D (almost never) I just adjust my UF goal by say 200ml to account for the fluid I'm taking in.