user expectation – How do I best make a tally of items on a mobile phone

I’m designing a wireframe for a mobile app for logging materials.
One of the features needs to be able to log the makeup of a material.

fx in the case of a technical fabric the composition can look something like:

30% WO, 30% CO, 20% AC, 10% PU, 10% WS

tallying to 100%

My main thinking sofar has been making an add button at the button at the bottom of the screen and then have the user add each individual part until the full composition is reached.

However where I’m unsure is how to make sure the full tally is 100% – not more not less, what sort of feedback?
But also how I would lay this out on the phone screen if I use a list and an add button?

Does anyone know of any apps that deals with something similar or know best practise for something like this?