Three Mayer, Brown LLP partners are the key lawyers steering cardboard manufacturer Packaging Corporation of America through its $1.27 billion acquisition of Boise Inc. The companies place the deal’s total value at about $2 billion, when $714 million of Boise’s outstanding debt is factored in. Boise Inc., based in Idaho’s capital city, makes packaging and paper products. Packaging Corp., based in Lake Forest, has been a Mayer, Brown client for the past several years, said partner John R. Sagan …