How is the physical alignment of the head determined during the bowing posture (Ruku')?

Step-by-Step Description of a Complete Prayer (كيفية تركيب الصلاة)

• The head should be raised significantly higher than the back.
• The head must be dropped lower than the spine.
• The back must be flattened and the head must be level in line with the buttocks.
• The posture does not have any specific rules.