How do you measure elbow room? Is it the diameter of the table space allocated to a guest minus their diameter? The diameter of the space occupied by all of a g

How do you measure elbow room? Is it the diameter of the table space allocated to a guest minus their diameter? The diameter of the space occupied by all of a guest's cutlery and plating subtracted from the table space allocated them? Something else entirely? Furthermore, if the diameter of a guest is to me taken into account in calculating elbow space, what is the preferred method for determining a value to be used? Surely not waiting for guests to arrive before measuring them. Perhaps measuring guests and taking the mean or median for future decisions regarding table sizing? Or perhaps choosing a guest diameter that maximizes your expected utility given previous observations of guests past and their reactions to different quantities of elbow room.