Since you need a 590mm circle of illumination for 12X20 you determine the minimum focal length required to cover 12x20 using the following formula. To use this formula you must know the angle of coverage of the lens.
Originally Posted by Gary892
FL = (590 / 2) * (1 / tan(coverage_angle / 2))*
which is derived from the simple relationship:
tan(coverage_angle / 2) = (image_circle / 2) / FL
Lens specifications are mostly useless in that they are not made with contact printing in mind and often don't indicate as much coverage as the lens will really provide. A case in point would be the 450mm Nikkor M you mentioned. As I recall the angle of coverage given for this lens is about 65 degrees and if you do the calculations above based on 65 degrees the coverage indicated will not be nearly enough for 12X20, but it is a well known fact that the Nikkor 450mm covers 12X20 with many inches of movement.
The only modern lens of 210mm that will cover 12X20 is the 210 SSXL and its angle of coverage is something around 120 degrees.