I have huge horizontal cameras here, but what the manufacturer of my camera suggests when micron tramming isn't an option(special $$$$$ equipment which guarantees that corner to corner the negative back is exactly parallel to the copy board) use a premanufatured grid like the translucent mini-cutting pads at craft stores. Now not only can you take clean measurements, generally at an enlargement so the errors are exagerated, I use a 1/100 inch ruler to be sure all lines are equal, also check the diagonals, and finally you can calibrate % of enlargement or reduction.