HTURKAK - I must be misunderstanding the question or your answer. Bottom and roof plates are typically lap-welded and no gap would exist between the plates. I don't see the gaps you mentioned in figure 5.3a - what version of the standard are you using, perhaps you can post the image?
Prashanth0206 - If by "nesting" you are referring to the gap between cuts on a CNC laser, waterjet or plasma table when the sketch sheets are cut from raw plate, API 650 is silent on this because it does not matter to API 650 how they are cut. I try to leave an inch between cuts when I nest sketch plates for fabrication, but it depends to some degree on the actual machine, software, process and operator being used for an actual minimum.