I am working with several probiotic strains to test the therapeutic effects of their cell-free supernatants (CFS). Most research methods involve treating mammalian cells (for example) with the probiotic CFS at 10% v/v (e.g., for 200uL total, 180uL of media is used + 20uL CFS). However, I cannot understand why this 10% concentration is the standard. I am wondering if it is based on anything so I can decide whether or not to test lower/higher concentrations. Please help!!