Referencing nodes in the expression editor

I'm trying to assign a constant head boundary that increases as a function of depth to a border with varying elevation and using the expression editor to do so. The equation I'm trying to input is

head = 600 + 0.0035(E[sub]Top[/sub] - E[sub]Current[/sub])

where E[sub]Current[/sub] is the elevation of the node the head boundary is being assigned to, and E[sub]Top[/sub] is the elevation of the node in the same location but on the top slice. I'm able to use the expression editor to reference the current node (using Mesh>Node Date>Current node number), but am unsure how to reference the node on the top slice. Is anyone familiar with the syntax the expression editor uses who would be able to shed some light on this?

Thanks

The quickest solution to this would probably be to copy the top node elevations to ALL nodes of a nodal User Data distribution and then use the value at the current node in this distribution.

Aside from this workaround, and assuming that you're working in a layered model, there is a way within the expression editor as well: There is a template for getting the value at a specific node index (indicated by brackets in the toolbar, check out the tooltips). With this you can get Elev(node index). But what's the node index of the top node corresponding to your current node? As FEFLOW numbers the nodes slice by slice and in the same order in each slice, you can get the node index of the top node by current node index (Idxn) minus (Current Slice Number Nos - 1) multiplied with the number of nodes per slice Nonslice. All the respective numbers can be found in the Mesh - Node Data section on the right hand side of the dialog.