We are installing a new microwave over our stove. We wanted more clearance above the stove, so we raised the cabinet shelf that we will use to secure the microwave. Now we need to adjust the vent pipe to make it shorter. We can't find information on this in any of our books. Is it so trivial that no one bothers to mention it? Our current vent pipe is 6 inches in diameter. They didn't have that size at Home Depot. Can anyone offer any advise? Thanks in advance.