I am not sure I understand your question right but it sounds to me that you are calculating the zoom ratio from the actual setup, where you need to calculate it for the projector alone.
Usually I do it by setting up the projector at a known distance from a wall, after having adjusted the focus to the distance needed for the actual setup.
I make sure that the projector is completely horizontal and that the image on the wall besides being a bit out of focus is a rectangle with no skewing or the likes.
I also make sure that any kind of keystoning is off.
then I measure all the figures in the projector node, the graphical representation of the projector is more important than you might think since it changes where the lens is placed in 3D.
The numbers that I have experienced most people having trouble understanding are zoom ratio and lens shift.
You measure the zoom ration by measuring the distance from the projector center to the wall and then divide it by the width of the image. use a full white image for the measurements and also preferably if you are using vga, use the same graphics card at the same settings as you will have in your final setup.
then divide the distance with the with of the image. be as precise as you can when the image is a bit blurry.
The lens shift is a measurement of how far up or to the side the image is “shifted” from the center of the imaginary line coming out from the center of the lens, the optical axis. if the optical axis hit the exact center of the image, the lens shift is 0.
If not you need to measure how much it has been shifted in either direction. You do this by measuring how far up or down it has been shifted and dividing that number by the height of the image. After that you do the same measurements horizontally, even though the projector says it has no sideways lens shift, it often do. Again measure how far left or right the image center has been shifted and divide this number by the width of the image.
These measurements will give you a good ball park starting point for fine tuning in your final setup. The factors that has the biggest impact are lens shift and zoom ratio, you can see when the rest are almost there, but those two can get you way off. while adjusting note the calculated numbers to be able to begin over again.
It is pretty tedious work and I know that there are people working on a solution to make this setup quicker… I have no idea about the time frame for this though.
one thing you can consider is that if your projection surface is flat, which it sounds a bit like from your question, you could use the Homography (Transform 2d) node to adjust the image to the projection surface… once you grasp it, it takes seconds to adjust to the surface. make sure to check out the help file, you can also just copy from there.
I hope it helped out