In my family (parents and 5 siblings) I’m the only one that never needed glasses. But for some time (I’m 79) I do use over the counter reading glasses for close work. I use +1.75 for using the computer and I just went up to +4.0 for reading. However when doing research I go back and forth from the screen to reference books and have to switch glasses which gets real tedious. I was thinking of buying some bifocals online to try them out. So how do I spec the specs? +1.75 for the main lens and + 4.0 for the additive, or +1.75 and +2.25 for the additive?
Have you thought about getting checked by a professional and using an actual prescription?
I had bifocals and hated them. I now wear contact lenses and put on readers to read things up close. The other option was a bit harder to manage - I could not figure out a way to make my arms longer.
I use these. Mine graduate from 1.25 to 2.5, they go up to 1.75 to 3.5. There may be other brands with stronger option:
The problem with ordering custom is that any place I’ve checked wants a prescription less than one year old.
“You haven’t got a baboon in your pocket, have you?”
There are several kinds of bifocals now. The old traditional type with two separate lenses in one frame, progressive lenses without a line or distinction. This is one of those times you really should go to the eye doctor.
Now, I have a very old prescription that I use to order my glasses. The places on the internet do not care about the age of your or acuracy of your prescription, they will send you a pair. I have even tweeked the range, orbit, etc, and ordered glasses. There are places, where the glases are so cheap that it doesn’t matter.
New Bifocals if you haven’t had them before is one of those times you really need to see the professionals.
There are very few products that I have owned in this life that have given me as much satisfaction as my progressive lenses. Some people find them a challenge to adapt to, but I took to them immediately. I’ll say that you get what you pay for, IMO of course, and that the cheaper ones are a total pain in the ass. Narrow vision corridors, unpleasant distortion of straight lines, generally hard to use. I would have thought that lenses were lenses, but nope, not my experience.
Agree on all counts.
Cheaply made ill fitting glasses are nothing like optically perfect glasses matching your measurements despite having identical prescriptions.
Hey, it’s genetics. 
I’m in the camp of getting to an optometrist.
Those look perfect!
It’s 1.75 plus 2.25 for the additive.
But i agree that it’s worth going to a professional, especially for your first pair. You want to get the distance between your pupils right, and there are probably other things. Try an optometrist. Their specialty is during your with suitable glasses.
Likely your vision is not identical between eyes. One eye may need a stronger magnification than the other. In addition, you may also have astigmatism, which is an imperfection in the curvature of your lens. The benefit of an eye exam is that it will give you a prescription to correct these issues much better than readers will.
Some of the discount eye glass stores, like America’s Best, have deals where you get an exam and two pairs of single-vision glasses for $100 or so. For a bit extra, you can upgrade to bifocals. Before you go in for your exam, figure out the distance from your eyes to your computer screen so the doctor can create the prescription for that distance. Readers will be a standard distance, but depending on how far you sit from the monitor, the computer vision prescription will need to be based on your distance to the monitor.
Repeated for emphasis. My vision was fine until I hit about 45, and then slowly got worse. One eye needed more correction than the other, though that has switched (!) over the course of the last 6 years (normal aging brough one eyes distance correction closer to the norm, and the “better” eye just got the age related loss of close up).
Another advantage for bifocals from a dedicated store is you can within reason determine the height of the bifurcation - I still can drive and read without them, but it’s far less eye stress when I do, so my reader/close vision is a few degrees lower than the default. I still mostly use them for night time driving and the rare in-theater movie because it’s much harder to compensate when it’s dark. YMMV of course.
Though if cost or insurance (or lack thereof) is an issue, that is a very different set of choices for the OP.