This is a good question......of course I define a good question as one which has puzzled me in the past or present. lol Actually, there is more than one correct answer or I should say there are several different reference points/external landmarks in the literature for proper icp monitoring/transducer leveling.
I would certainly check your facility's policy and procedure manual first. If the matter is addressed, I would use that. If not, perhaps you and some others might work to write a policy. If you are doing ICP monitoring, you need one.
What has worked for me is to ask the neurosurgeon at what level he or she desires the transducer. Then I clearly/prominently document the position where the measurements are to be obtained.
This serves 2 purposes: If the surgeon is a stickler for specific landmarks for different catheter tip positions, you will know it. Secondly, you will achieve consistancy in your readings between nurses which is extremely important yet apparently lacking at present.
As noted above, if specified I would definitely use the facility's written policy. If you are asked by the surgeon to deviate from that policy, I would write that as an order---in turn the order should be transcribed to the Kardex/computerized patient profile or the like to insure accurate communication between nurses.
Probably not the concrete answer that you were looking for, but it has worked for me. I do think the generic answer is between the top of the ear and the corner of the eye.