Thanks! We initially put together this calculator for that very reason, having to work with a few different small proteins and peptides without tyrosine or tryptophan residues. I am not familiar with A205 measurements specifically on peptide caps, and I haven't done those measurements myself, though I think the best guess would be that an N-terminal acetylation would have the same effect as adding a peptide bond (2,780 M-1 cm-1), and a C-terminal amidation would have a similar effect to adding a glutamine or asparagine side chain (400 M-1 cm-1). The effect of the C-terminal amidation is so small that it could be ignored, but for the N-terminal acetylation, you could either add 2,780 M-1 cm-1 to the calculated extinction coefficient or just add a small amino acid (e.g. glycine or alanine) to the inputted sequence. Any larger caps or modifications (especially involving any aromatic groups), though, would have a much greater effect on the extinction coefficient.