Proper hemping of a drone reed seat is just about patience and getting the hemp to conform to the exact shape of the reed seat. Take your time and add hemp where necessary. Notice that as it gets to the right thickness it takes on a shiny/darker look. This happens when the hemp is compressed and indicated that it's contacting the inside of the reed seat.
This post is written by Dr. John Holcombe.
One of the biggest fears some pipers have is that one of their drone reeds will suddenly come out of its reed seat and fall into the pipebag, leaving you with no ability to play. If this ever happens, trust me here, it will occur during one of your most important performances. But there is a way to totally eliminate the possibility of a drone reed becoming dislodged, and that is to “thread” the reed seat using a commonly found tool.
Have you ever felt that you were struggling with your pipes, or that they were too hard to blow, or that you just couldn’t blow enough air into the bag to maintain the correct pressure? Can you play for no more than 10-15 minutes, even with an “easy” chanter reed? Have you answered "yes" to any of these questions?