The best formula for calculating protein concentration is the Beer-Lambert law, also known as the Beer’s law. This expression is:

A = εlc

Where

• A = absorbance
• ε = molar absorptivity or molar extinction coefficient which is a measure of probability of electronic transition
• l = path length
• c = concentration of the protein

References:

Grimsley GR, Pace CN. Spectrophotometric determination of protein concentration. Curr Protoc Protein Sci. 2004;Chapter 3:. doi:10.1002/0471140864.ps0301s33