What's UA policy in calculating the miles required for upgrading?

For example, when I will be flying BOS-LAX this Christmas season, I requested to use the certificates and was told 3000 miles (6 cert.) will be needed. However, the flight is only 2600 miles!?

Since I rather spend less $$ out of pocket, especially if the BOS-LAX trip will require 12 cert., I will likely use the 20k miles to upgrade.