With the BCA assay you are measuring the absorbance. You need standards to make a standard curve from which you can interpret the concentration of protein in your samples. In Excel plot your absorbance along the x axis and standard concentrations along the y axis. You can then use the Format Trendline option to display the equation. It is advised to also try 2nd and 3rd order polynomials to see what best fits your data (see how well the predicted protein concentrations of the standards based on the absorbance correspond with what is expected). Assuming you are using a linear model with m = 0.0377 and c = 0.0798 and BCA assays typically measure in µg/ml, you would need to concentrate your samples by over 3 orders of magnitude to obtain 20µg/µl. As you have not shown or described the concentrations of your standards I cannot determine if you have already factored in the differences in magnitude. Assuming your values are in µg/µl you would need to divide 20 by the protein concentrations in your samples to determine the number of µl you will need.

Robert Adolf Brinzer - Yes, I have already accounted for the differences in magnitude in my formula. After some additional calculations, I came up with this new table. However, I am still unsure if I calculated the final concentration correctly?

As all your samples are around the concentration of your 12.5 standard their absorbance should be around that value.

Without the raw data I cannot say for sure if you are crunching the numbers correctly but the final values seem reasonable for a sample.