What is the ideal height of the radix region in rhinoplasty in men?

Radix height is more of how it relates to the rest of the nose than an absolute number, particularly its relationship to that of the tip. In thick skinned noses with only augmenting the radix, an implant can be  very effective and safe if one does not want to use their own cartilage for the augmentation.

 A closed rhinoplasty procedure can address a low radix by adding a piece of cartilage harvested from inside the nose, or in a situation where the nose has been  depleted of cartilage, an ear cartilage graft could also be used. The height of the radix is  dependent up on the height of the tip and  bridge line and any hump that is present  and subsequently removed. For many examples, please see the link  and the video below

From the wording of your question it seems like you have a strong understanding of nasal anatomy.  Using the medial canthus of your eyes, the inside corners, will allow one to assess radix positioning.  If the angle or take off point of your nose lies below the canthus one can say it is a low lying radix, as it gets closer to your forehead it becomes higher lying.with respects to its height it should be roughly a third of the nasal tips projection.   Some suggest using an isolated graft to augment the radix.  One example being your own cartilage wrapped in your bodies fascia or "natural Saran Wrap".you can use a silicone dorsal onlay graft that spans the entire dorsum up to and including the radix.  This allows for increased height and a higher radix take off point, closer to the forehead.   A thicker skinned patient is a more ideal candidate as this decreases the feared risk of extrusion of the implanthope this is helpful regards