With this tool you can calculate your exposure and needed adjustment for development. It uses zone system theory originally invented by Ansel Adams and Fred Archer.

First decide zone of your shadows and measure the shadows. Then device highlight zone and measure the highlights. Measure these with the film ISO you are going to use. You also need to decide shutter speed in beforehand.

The app calculates exposure aperture for you and also tells if you need to shorten (N+..) or expand (N-..) your development time.

You will get best results with a spot light meter but mobile apps will probably do fine.