To put in the simplest way, you need to have been prescribed an Rx from your eye doctor with an add power. This presbyopic prescription is then converted for a mid length prescription. For a computer screen, or arms length distance, you would generally use half of the add power prescribed on your Rx for the computer glasses.
An example of an Rx written might be:
O.D.= SPH-1.00 CYL-0.50 AXIS 90 ADD +2.00
O.S.= SPH-1.25 CYL-0.25 AXIS 75 ADD +2.00
You would then take half of the add power +2.00, which would be +1.00, and then add it to the SPH (or Sphere) of the prescription.
The add power is generally calculated for a distance of 12 to 18 inches for reading. half of that power would be for approximately 24 to 36 inches of reading distance (about the average distance of a computer monitor) that prescription would look something like this:
O.D= SPH -1.00 + 1.00 (HALF OF ADD)= 0.00
O.S.= SPH -1.25 + 1.00 (HALF OF ADD)= -0.25
The prescription then would be:
O.D.= SPH 0.00 CYL-0.50 AXIS 90
O.S.= SPH -0.25 CYL-0.25 AXIS 75
The best way is to ask your eye doctor for a separate computer glass prescription to be written, but you could just give your normal prescription to an eyeglass shop and say that you want computer glasses. they will use this same basic way of estimating your prescription.