D/2 and the helpful video are just right! One small criticism in the video: although it probably would not be an issue on the size stone shown, it really is best to start the stone cleaning process at the bottom to avoid streaking. Once the lower areas of the stone are clean, debris and dirty water running down from the top have nothing to cling to and you're not left with streaks that are difficult if not impossible to remove. If folks are interested, The Association for Gravestone Studies at www.gravestonestudies.org is a great resource and Save Your Stones clearly is following all their cleaning information. (AGS has tips for making inscriptions easier to see when photographing headstones, too.) Thanks so much for this addition to your efforts!