Single liability duration match

For single liability using duration match. It is important to match duration of asset and liabilty and also minimize convexity. In some questions, we choose the one with the least convextity although the duration of the liability is a little bit bigger than that of the asset) to minimize structural risk while in some questions it states the duration is not closely matched so should not be the right one even though the convexity is the smallest one. I can see the duration match has higher priority then convexity. But how to define closely match (e.g. duration of liability- duration of asset <0.01)