At left: note how the towing point can be located on a standard delta plan by using a straightforward percentage of the length of the center sleeve.
The chart at right illustrates how the towing points of non-standard deltas can be thought of as equivalents to the standard delta's percentages for comparison purposes. The percentages shown are not the actual ones found on the kites, because most of those kites have other than standard triangular planforms, and the straight percentages do not apply. (Non-standard shapes include scalloped, clipped wing and extended keel deltas, whose towing points are worked out as percentages of the kite's wing areas, as shown here.)
The conversion to equivalents helps one to compare non-standard delta designs directly with the purely triangular delta standard. Recall that towing points forward of the norm reduce pull - lower lift is better for breezy conditions. Towing points rearward of the norm catch more wind, giving more pull, which is better in very light winds (but court disaster in strong breezes). Towing points toward the front make kites climb faster as well, but beyond a certain point stability suffers. Conversely, towing points too far toward the rear can make a kite reluctant to climb.
The "standard" or norm for a Standard Delta is a compromise, a blend of lift, stability and good handling. And at exactly half the center spine, a quarter of the wing area, it's a nice, round, easy-to-remember, almost magical number.