D2 is intended to be the distance to the buffy coat. So if you are using a gel tube (where the buffy coat will appear just above the gel) things get complicated. You'll have to break D2 down into its components and add them up. D2 has 2 components: (1) the distance from the top of the tube to the top of the column of blood, and (2) the distance from the top of the column of blood to the middle of the column of blood (before centrifugation). You can measure (1) directly on a filled gel tube. To get (2) you will have to measure the height of the column of blood in a filled gel tube and then divide by two. Then D2 = (1) + (2).
