Of course, a column length of over #15*m# is unrealistic; 1 atmosphere's pressure can only support a mercury column #"760 mm Hg"# high. Part of the reason we use mercury manometers (and mercury is (i) dangerous, and (ii) difficult to clean up if spills occur, which they always do), is that the column length that an atmosphere's pressure pushes up, i.e. #"760 mm Hg"#, is a very convenient measurement.
I can remember in the older days, that some labs had mercury manometers attached to a vacuum distillation. You could bleed in inert gas so that the vacuum was raised from full, and you could distill at a higher pressure, i.e. #"10 mm Hg"# rather than full vacuum.