Simple Explanation for Cropping

I have no problem coding a script to Open a PDF, acquire a page, create an AcroExch.Rect Object and issuing pdDoc.CropPages(nPage,nPage,0, oRect) - where oRect contains point parameters for top,left,bottom,right.

That is because those parameters were always known. Now I need to crop a portion like lower-right quadrant. The peculiarity is that CropPages() takes 4 parms while GetSize() returns only x and y [the width and length of the page in points - usually x=612, y=792 [for 8.5 x 11]. I (and please I hope there are others) need someone to go slow with me and say if you wanted to crop the 'crop me' area, how you go about determining top,left,bottom,right

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TIA
Stan