Apply Formula on Mail Merge Field

I would like to create a mail list with a calculated field.
I have a working mail merge document, except for one field.

In my case, I have 3 friends who go out for dinner. Each friend pays a % of the meal. Each meal differs in cost. I’d like to send a letter to each person with the amount of money they owe (“total cost” divided by “contribution %”).
I tried the following,
I went to Variables (fields)

It said “This option is only available if the “Insert Formula” field type is selected.”

I selected Insert - Field - More Fields - Variables - Insert Formula

I tried to enter something like “100 / <Contribution %>”, where <Contribution %> is a merge field and 100 is the total cost of the meal. So,
100 / 20 = 20
100 / 15 = 15
100 / 65 = 65

Ideally, at the start of the merge, it would say “What is the total meal cost?” and I would enter 100, then each letter would be sent out with the correct contribution ($20, $15, $65).

Is this possible?

I am currently using LibreOffice Writer 7.2 under Windows 10 and I’ve used other Office suites to do similar in the past.

Thank you

PS This is a repeat topic. I could not see an answer to the question.

If you mean by that you submitted the same question again, this is a bad idea: you’re creating confusion and possibly would receive partial (and perhaps contradictory) answers.

You didn’t include a link or reference to the initial question.

The trick to “refresh” a question and bring it back to the list top is to edit it. Even adding a simple space is sufficient.

Thanks for asking.
Someone else had previously asked the same question about a year ago.
They closed the question. There doesn’t appear to be an answer to this question yet.