Updating the tool offset for the currently-selected tool?

I've got a toolsetter probing routine working well enough, but I'm not sure how to get LinuxCNC/Axis to write the resulting value to the tool table (i.e. the same as clicking "touch off tool"). I've found numerous examples that assign to `#<_ToolRefZ>`, but I can't find any explanation of what that does.

How can I write to the current tool's offset in the table (such that it's persisted)?

If you work with toolnumbers, #5063, to make shure a tool is loaded and avoid errors and other problems in preview, or if the tool is loaded while processing, its a good start of the routine with

(Check for tool loaded)
o777 IF [#5400 LT 1]
o801 IF [#<_task> EQ 0]
o801 RETURN [99]
o801 ENDIF
(abort,NO TOOL LOADED... !!)
o777 ENDIF

and after probing

G10 L1 P#5400 Z#5063 or your Z calculation