size of windows opening.

Hi,
I am doing an extension, and need to make a new window opening in the wall frame. I want to use 1200mm blinds (which I can buy from bunnings for masters), can I work backward to calculate how big the opening should be? the new window is going to be KDHW, double glaze.

I contacted the window manufacturer, but the sales person has no idea, and their techie guy is on leave, but I need this urgently since my carpenter is waiting.

Many thanks
cxc


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