How do I size an investment within the portfolio?

I manage a concentrated portfolio of ten to fifteen businesses. While I spend weeks and months on a particular investment before I start buying, it is a fact of life that one would get to know a business more closely after one owns it. As I start building a position, my objective is to get that investment to about 5% of the portfolio. As I continue to learn more about the company and the industry, I’d look to add more.

My self-imposed limit on any particular investment is 15% at cost.