There is no specific dilution factor for Ab. Generally one has to optimize Ab based on the conc mentioned on data sheet. In such case I would choose an appropriate diluent, as recommended in the data sheet and make up 4 dilutions of the concentrate – 1/50, 1/100, 1/200, 1/400. Take four slides and commence a normal assay on all 4 slides. Review these slides and you will see strong staining to weak staining. Select an appropriate intensity that best suits your purpose and note the concentration of Antibody. Hope it helps