No need to complicate things. Meter the main light source. Take a reading and take note of it and turn off the main light. Turn on you fill, take a reading. If it's a 2:1 ratio the fill is one stop less than the main light. The fill ideally should be a broader i.e. a flatter light source. Don't get hung up with numbers and ratios. See how it looks to you. A dome type of incident meter works best with 3 dimensional subjects. No need for a flat diffuser for your meter if your doing portraits.