I don't know the formulas, but I don't think that's right. How can the height be 10'9"? A standard ceiling height is 8'. Maybe you mean width? Not trying to be grammar-nazi-ish, just want to make sure of what you're trying to say here. Also, the only reason I bring it up is, if you're sitting too close to a 150" screen, you'll see the pixels (even on the best projectors), and that's a big distraction from the image. I do not agree that bigger is better. Better IMAGE is better. If you took a standard projector and blasted it on the side of a building in the dark...sure it would work, but the image would NOT be great unless you were sitting so far away that it was practically the same as if you were proportionally sitting in front of a 120" screen in a basement. Don't get me wrong, I am NOT trying to say 'your idea sucks' or anything like that. I'm hoping I'm helping here. I've had projectors now since the early 2000's, and one of the early takeaways I have is that bigger is NOT better. I've found finding the sweet spot where your image is AWESOME, is best. Sometimes that can be a 96" screen. It depends on where you sit.For 720p the source will upscale it, so no issues.
The difference between an 120 and 150 is not small. Keep in mind that the height of an 120" (16:9) is 8.7' and 10.9' for 150". The bigger the better.
Just my two cents, which is what a forum is for, right? Hoping I'm helping.