Dushie,
I can assure you that the screen size of 144 inches wide is very big for your room size, the viewing distance should be around 30/35 feet to view the screen size of 144 inches, while your viewing distance is around 21 feet, this is a huge gap.
For your room size, the screen should not be wide by more then 72/80 inches, the calculation which you are saying i do not understand that, please take a opinion on this from some reliable source.
If you are planning to buy a dalite screen i can give you a source, you can get a good price from them, once you have this price, you can compare it with others.
The colours which you will choose for the roof and walls, it should not be bright, how about going for wall paper. Please consider blue colour as most of the theatres fabric walls are of blue colour.
My mini theatre walls are of fabric. I had bought several colours of fabric, each colour fabric was 4 feet wide, and the length was around 8 feet, i used to put these fabrics on the wall and observe for a few days, and then out of that finally i choose the red fabric for my theatre. At one point i had approved some brown type fabric, i bought for the full room, and then after putting it i realized i have not liked it, so that full money in-directly went down the hill.
Vinay.
