There are all kinds of fancy modeling tools out there that are designed to help you model existing objects. There is this great one for Blender that lines up a photo to your model and helps you draw the model right over the top of it. In the end, however, nothing seems to work as well as just counting the stones on the wall.
This update to Helsinki Finland Temple was just that. I spent the first of my time adding details, then I started adding textures. The textures showed me my scaling was off and I spent much time reworking the model to better proportions.
The entryway and the spire still remain, as do the Celestial Room windows (The large section at the opposite end from the spire.) Still, moving along rather nicely now, and I have begun shaping the grounds as well.