I think you would need to do two steps…
Input Variable (Formula to Execute): Formel from first example, or “VBEZ*12/100” from second
Output Variable (Variable to Set): BBRV from first example, or ZVBEZJ from second
Then your template would need to call an evaluate on the Input Variable as part of your worker/function.
In theory, the resulting code would be more like:
<zeebe:ioMapping>
<zeebe:input source="=84600" target="temporaryVariable" />
<zeebe:output source="temporaryVariable" target="BBRV" />
</zeebe:ioMapping>