Q: How low do I have to bow when making ruku?
A: The minimum required position for ruku/bowing is to bend low enough for the hands of a man of average height and size to touch his knees. It is however preferred to bow low enough for your back to be parallel with the ground. Source: Sharh al-Muntaha
This answer was collected from HanbaliDisciples.com. The questions have been answered by Imam John Starling.