You can't use cardboard or anything else. You actually have to put it up and watch TV on it. This is a fallacy of human perception. Thinking you can throw some measurements up and know how your eye is going to work. You just can't.\n\nIn time after time after time after time, the number one complaint people have after buying a TV is that they wish they had gotten the next size larger.\n\nI've convinced clients for years to get a bigger TV than they wanted to buy, and often I'm not selling them the TV in the first place. So, no profit for me on the TV sale. The goal is to get the right size for the viewing distance. At 9', you can easily support a 75" display. It takes a few hours of viewing before you wondered why you worried about it being too large at all. I've had weekend installations where people have thanked me before I left for insisting they purchase much larger than they had planned for. I have NEVER had someone wish they had moved to a smaller size ever.\n\n\nThis couldn't be more true. Last January I replaced my 65" Panny plasma with a 77" LG CX. Primary seating is about 12 feet away. Now I wish I would have gotten the 83".