Filling Schedule Gaps

Hey guys,
Here’s what I’m trying to build and I’m hoping you can point me in the right direction-

Say I have a schedule that has missing item numbers in it (type mark). I also have a specific family that I want to be copied and then numbered to fill in those gaps. I assume step one is to find the missing item numbers. Then step two is to copy the family (I would assume an array here since we want them to not be on top of each other). Then step three would be to number them using the previously discovered missing item numbers. Does that sound correct?

Now, let’s start with step one. How do I use Dynamo to find those missing item numbers in my schedule?