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The pasting the formula into the formula builder that appears

Displaying generated map scale

As of version 2.1.0 of the com.cohga.server.report.birt.map bundle (Weave 2.4.3) a new parameter is available, after a map component is generated, that provides the value of the scale that the map was actually generated at. The parameter will be named scale, if the map doesn't have a name, or <name>.scale if the map does have a name.
The numeric value is directly available as params"scale" (or params"<name>.scale") or a formatted string value available using params"scale".displayText (or params"<name>.scale".displayText).