How to calculate the cost and time of work of a freelance designer / agency?

Question

Freelance

Tasks are always different, hourly pay depending on skill level and greed (freelancer/agency) is ok.
I want to figure out how to calculate my hourly cost and time to complete a job.
Landing - X hours.
Corporate website - X hours
Mobile application - X hours
An order form arrives to a freelancer or agency (outside of freelance exchanges), the volume, terms and cost are discussed. The customer makes some prepayment or the full cost. For non-trivial tasks, the terms may increase, then directly tell the customer: "Bro, you still have 20 hours * X $" ?
Thank you.

Answer 1

Write a clear TOR + add a certain number of hours for revisions during work, if not enough - discuss the cost individually

Answer 2

No way.
In addition to the landing page, everything else can differ significantly in time, even dozens of times.
Landing can be priced at a fixed amount.
Everything else - nothing.