Note that I'm not an optical professional of any sort. Use this information at your own risk, etc.
If you're nearsighted like me, this is what I would do: Put your glasses on in their normal position. Use a millimeter ruler to measure the distance between the bridge of the frames and the end of your nose or some other convenient benchmark. At the computer with your head positioned where you want your new lenses to work, pull the frames down your nose until things are sharp. Measure the bridge to nose or whatever distance again. Put the difference of your 2 distances in meters plus your original lens power into the vertex correction formula:
Fc=F/(1-xF), where Fc is the power corrected for vertex >distance, F is the original lens power, and x is the change in >vertex distance in meters.
A message in this thread explains how to handle the sphere and cylinder:
The exact formula for vertex distance compensation is:
New Power = Old Power / (1 + Change * Old Power)
where Change is the increase in vertex distance in meters (use >a negative value for a reduction in vertex distance.
The approximate formula is:
New Power = Old Power - Change * Old Power2
For lenses with cylinder power, the power of each principal >meridian should be compensatedly individually (that is, >compensate the sphere power and then the sphere + cylinder >power). The new cylinder power is the difference between the >two compensated principal meridian powers.
For instance, given a prescription of +4.00 -1.00 x 180 (+3.00 >through the cylinder meridian) with a refracted vertex of 13 mm >and a fitted vertex of 20 mm, which represents an increase in >vertex distance of 0.007 m, the new compensated prescription >should be:
New Sphere = 4.00 / (1 + 0.007 * 4.00) = +3.89 D New Cylinder Meridian = 3.00 / (1 + 0.007 * 3.00) = 2.94 D New Cylinder = +2.94 - 3.89 = -0.95 D New Prescription = +3.89 -0.95 x 180