You could use a constant sum matrix question, with the sum set to scale points.
Then use Math operations to get the proportion.

Thank you Ahmed !!
Those answers to Q1 will come from different respondents (I'll have several lines in my data table).. is the sum matrix still work in this case to combine the values from different persons?

I even tried to pass the answer from one person to the URL of the survey, and use embedded data to store it. But it doesn't work, because when the second person opens a new survey, the embedded data I've saved from the previous one is lost :(

All that I want, is to create a variable to sum a column from all the response lines

https://www.qualtrics.com/community/discussion/comment/37880#Comment_37880The answer I've provided does exactly this.
Now your comments are about adding multiple people and multiple questions. So, I have no idea what you want to do or what your config is.
The best way to get quick help is to share your problem in as much detail as possible in one post. If details are confidential, then send a PM. If even that is an issue, then you're better off hiring a developer for this.