Generating an alpha channel for document

Hi.

I'm trying to generate an alpha channel based on the transparency of all the layers in the document.

I usually do this manually by going into Image->Calculations, selecting Transparency in the Merged layer's channel and setting the blending mode to normal.

However, I can't find a way to invoke the calculations window and it won't record in ScriptListener.

So can someone please help me with invoking the function with these specific options or suggest another way of doing this?

For now I've done this by merging the layers and selecting the transparency of the resulting layer but I would rather keep the layers as they are and avoid unnecessary calculations.