Ditto, to Fabrizio's advice. You didn't mention if you are using studio lighting that you control or available light or mixed lighting sources. With varying light sources and subjects your settings will have to match the situation. IMO you don't want to over expose but rather to properly expose for the particular situation. My limited experience with Portra has been to get excellent exposures at box speed with incident readings and available light and when necessary reflectors to open shadows.