DWB, I suspect I may be one of your Luddite bretheren.
I don'y know the answer to your question but it occurs to me that if modern aircraft don't fly in accordance with the Standards set by ICAO then the dimensions of the holding area and separation between holds may not provide the space between aircraft that everyone thinks there is.
Think about that next time you're cleared into a hold under a procedural service. And yes, aircraft still get procedurally controlled sometimes.