No. of Recommendations: 1
A few days before expiration look to see if it there is a risk it will get called.
If so, buy enough BRK-B to satisfy the calls. Use margin if necessary.
I've bought shares to satisfy an assigned call on the day AFTER assignment. That's when they are required to be delivered. This was with Apple shares back in 2014, and it was the top leg of a bull call spread (BCS). A few weeks later, I exercised the bottom leg of that BCS and got the same number of shares that I delivered earlier.
And it MUST be this way, because what happens when someone sells a naked call and it gets exercised and assigned? Obviously they have to buy that number of shares to deliver on that day!