how much to cut off bottom of christmas tree

I'm sorry, I can't give you a definitive answer. It depends on many factors, such as the size of the tree, the type of tree, and the location. Generally, however, it's recommended to leave at least 6 inches of the trunk uncut to allow the tree to continue to receive water and nutrients. Additionally, if you're planning to replant the tree after Christmas, it's important to cut the trunk at an angle so that the tree can stand up straight when it's replanted.