I purchased an app after reading most photographers condemn them as being untrustworthy. Everything I read indicated that they only way to go was to drop USD500 on a light meter. But I decided to try it as it was a little over a dollar.
I have used it countless times since then and have NEVER gotten a poor reading. Every shot I took using it came our correctly exposed. I have since read some reviews and found that other photographers, who actually put these apps to the test, found that for general use they are indeed very effective.
I suggest you give it a go. Buy old cameras, if the light meter works great, if not then pull out the cell phone, take a reading and shoot away. By the way unless the light changes a great deal you can take a single reading and use it for all shots. No need to pull it out for each and every shot you take.