Mahr, and how to write it down.
There is no formula for the amount and we are not going to invent one. What most families actually lack is the vocabulary to discuss it and a record of what was agreed. Five questions, then a form of words you can take to the nikah.
This is general guidance, not a fatwa. Where something is disputed, ask a scholar you actually learn from.
Who is deciding the amount?
The mahr is her right and it is hers alone. It is not paid to her family, and it is not theirs to spend.
Is it paid at the nikah or deferred?
Both are valid. What is not valid is leaving it vague, because an unwritten deferral becomes a forgotten one.
What form does it take?
It does not have to be money. It has to be something of value that she has actually asked for and can actually receive.
Is anyone under pressure over the amount?
This is the question that matters most and gets asked least.
Is anything being asked of her family?
Dowry demanded from the bride's side is a separate thing from mahr entirely, and it has no basis.