Paint on a mesh and calculate the percentage of what is painted

Hi there.

I'm trying to make this. 1. Paint a texture of a 3D mesh in real time 2. When you paint the mesh you can calculate the % of that you have painted, something similar to this

Someone has an idea o something I've been stuck in this like for two days. Thanks in advance.

Link [1]: https://play.google.com/store/apps/details?id=com.mulberrysoft.BRF_Android&hl=es≷=US