That should be feasible: is there no free FITC label present in the sample? And do you know if the peptide has been labeled uniformly with one FITC label per peptide?
Are you planning to determine the concentration based on a FITC (free label) calibration curve?
Yes you can measure the absorbance and calculate the concentration by rearranging the standard formula A=ecl. You do not need a calibration curve, because you can use the known molar extinction coefficient for fluorescein, e=70000 M-1cm-1, in the equation. Path length for a nano drop is about 1mm, and you’ll need to ascertain if the software multiplies the measured absorbance to compensate. I think it probably does. In terms of accuracy, however, it would be far better to measure the absorbance in a cuvette, which will have an actual path length of 1cm.