That's what you think. Your problem is that you believe a lens' actual focal length is the number engraved on the trim ring. Equivalently, you believe that the flange-focal distance of your lens is exactly the flange-focal distance of the lens that belongs to the person who advised you. This is rarely quite the case. You have to measure a lens focal length before making fixed focus camera to use it or you have to be able to focus the camera through the lens so that you can set the lens-to-film distance so that it focuses at the desired distance. Or beg/borrow/steal a view or press camera to use to measure flange-focal distance at the desired focusing distance.