Sorry about this, but we have not yet implemented a solution to this. Our original implementation had "z" as a proxy for z+1 until someone told us the calculations were off by 1 and we "fixed" that. Later we learned about z and z+1 as separate options, and as you say sometimes z+1 is more present.
So, it seems that we will need to add a way to specify both z and z+1 as ion type choices. Perhaps adding z' or z* or something to our nomenclature for ion types in the Transition Settings - Filter tab.
Any further thoughts you have on the subject would be welcome feedback. We don't get a lot of requests for this, but it has clearly come up a few times in the past few years.