How to calculate interior and exterior finishes estimates efficiently?

I am working on a project where I need to make very accurate interior and exterior finishes estimates. I want to understand best practices from consulting perspective, how to approach estimation problem in professional way. For example, when I calculate cost of wall painting, flooring, and facade finishing, sometimes numbers are very different from client expectation. How can I structure estimation in method that is clear, reliable, and easy to explain to client? I also want to know if any consulting frameworks help to analyze cost breakdown for both interior and exterior finishes. I try to combine consulting thinking with practical construction estimation. Any advice, examples, or step-by-step method will be very useful. Thank you in advance!