Ivan Vegvary writes:

I want to order some bifocals, and obviously I will use the same
prescriptions.
OD SP +0.75, Cyl -0.75, Axis 098, Read Add +2.5
OS SP +1.00, Cyl -0.50, Axis 110, Read Add +2.5

However, I would also like to order a pair for the computer. I use
+1.5 reading glasses for the screen, and switch back and forth to +2.5
reading glasses for reading. If I want bifocals that do the same
thing (obviate the switching) what numbers do I use? Where do I plug
in the +1.5 value?

Change both adds to +1.5, instead of +2.5.

Why: The spherical/cylindrical lens should correct your vision for
focusing at infinity. The additional positive refraction (the "add") is
specified in diopters, which is the inverse of the focal length in
meters. If you hold a lens of power d in diopters in front of your
infinity-focused eyes, you will then focus instead at 1/d meters. So a
+1.5 add will focus at about 1/1.5 = 0.67 meters, or about 2 feet, which
for most people is a suitable computer screen reading distance. Your
2.5 add let you read a book 1/2.5 = 0.4 meters, or about 16 inches, from
your face, suitable for books.

So whatever distance you want to sit from the screen, in meters, invert
that and specify it as the "add" number.

Rather than bifocals for the computer, you might consider single-vision
lenses so you're not tipping your head about to read the screen. For
that you would specify no add, and instead add 1.5 to the spherical Rx
numbers, that is, 2.25 and 2.50 instead of 0.75 and 1.00.