You brought back memories of (long ago) high school. That math formal where x/x = y/y... take a yardstick, measures its shadow. Then measure the objects shadow. Plug numbers into the fractions and easily calculate the missing variable.
However, there is a much easier way with the Phantom. You cannot run a POI mission in the Go app without setting it up manually. You will also want to frame your shot in realtime. There is really no need to estimate a height - just set the bird to frame your shot and go from there.
If your concern is RTH height, you can set a high value and then adjust after flying and reading the altitude values.
If you really need to guess, a good rule of thumb for buildings is 10ft per story. A 5 story building will be about 50ft tall. That is a generations and if you are flying in some autonomous mode, you should really use a more accurate tool (such as personal observation while flying and getting the readings from the app).