For the Standard 3.1 mL Assay and 96 Well Plate Assay, the protein standards should be prepared in the range of 0.1 - 1.4 mg/mL. This is not the final concentration of the protein after being diluted in the assay.
For the full protocol, please refer to the document found here:
https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/165/479/b6916bul-ms.pdf