When peptides are reconstituted in 200ul water, the concentration would be also 0.5mg/ml. To start, the best ratio would be 1:1 (which means molar excess of peptides relative to antibodies when identical volumes are mixed). Please mix equal volumes of peptide and antibody at the required dilution and leave at ambient temperature. Best is to have two identical blots, to be incubated with equal amount of antibodies, but one with the antibodies pre-adsorbed to the peptide for 20min. Then incubate and develop in parallel the two blots.