When using a 10 mm path length,  simply multiply the OD by the conversion factor to determine the concentration. Example,  a 2.0 OD dsDNA sample corresponds to a sample with a 100 µg/ml concentration.
When using a path length that is shorter than 10mm, the resultant OD will be reduced by a factor of 10/path length.   Using the example above with a 3 mm path length,   the OD for the 100 µg/ml sample would be reduced to 0.6.   To normalize the concentration to a 10mm equivalent,  the following is done:
0.6 OD X (10/3) * 50 µg/ml=100 µg/ml
Most spectrophotometers allow selection of the nucleic acid type and path length such that resultant concentration is normalized to the 10 mm path length which is based on the principles of Beer's law.