Possible options and issues:
*Find when there are '''first ''' two hulls and then step back a layer -- but there might never be two hulls, so if there is only ever one hull then find the max layer.
*Form a chain from layer 1 on down that breaks when there is no longer just one hull. Perhaps count the number links, grouping where the chain has one hull or not, or require that the chain contain level 1... [https://jaxenter.com/10-sql-tricks-that-you-didnt-think-were-possible-125934.html]