Happy to help given I'm ex-BCG myself!
There are a few answers here.
- If the item is quantifiable (i.e. market growth), then the answer is easy. The middle point in the data is the middle point on the matrix. So, if you have 5 markets, with growth of 1%, 2%, 5%, 6%, and 10%, respectively, 5 is the middle and 10 is at the top!
- If the item is not quantifiable then it's a judgement call. This honestly happens a lot. A consultant will put their best estimate on the page and Principals and Partners will review and adjust based on their understanding of the situation. In many situations, clients also provide their input during BCG-led workshops.
Remember, while this sounds like "guessing" it really isn't. You are paid to have a good understanding of the world and complicated, hard-to-make tangible information. You use the information available to you, make reasonable assumptions, and generally things are in the right place at the end of the day.