A Human Design chart is produced from two sets of planetary positions, not one, and the second is the more unusual.
The Personality calculation uses the moment of birth. It is printed in black on a standard chart and is described as the conscious side: what a person recognises about themselves and can talk about.
The Design calculation uses a moment roughly eighty-eight days before birth, fixed precisely as the point at which the Sun was eighty-eight degrees of arc earlier in the zodiac. It is printed in red and is described as the unconscious side: the body, the genetic inheritance, the aspects of a person that others observe before they do. Human Design associates it loosely with the formation of the neocortex during gestation.
Both sets of positions are converted into gate activations by placing the planets on a wheel of the sixty-four hexagrams, each hexagram occupying an equal arc and subdivided into six lines. A planet therefore activates a gate and a specific line within it, which is where the six-line structure of the I Ching enters the calculation in a substantive way rather than only as a source of numbers.
The two calculations are combined into a single bodygraph, so a channel may be completed by one gate from the Personality side and one from the Design side.
The eighty-eight degree figure is worth noting as an example of the system's character. It is precise, it is applied consistently, and no derivation for it is offered beyond the original transmission. It is not inherited from Western astrology, which has no such convention, nor from any of the other traditions the bodygraph draws on.