Need help with creating a symmetric map in the editor ?

Hi guys,

is there a way to make a symmetric map in the editor, where I start exactly from the centre of the map quickly ? Or do I have to count the grid tiles in the editor and work my way from there ? I imagine there is some copy/paste option, so I would only to one side and then copy it, but it I want it symmetric all needs to be counted well..is there a way ?

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Thank you :)