How the numbers change is going to depend on several factors.
For example, my bifocal Rx contains two sets of numbers. One set is for my distance vision, the other for my near vision. If I wanted a cheap pair of back-up glasses, for, example, driving, I could get single-vision lenses using just the distance numbers. Likewise, if I wanted glasses dedicated just to reading I could use the near vision numbers.
There are various configurations of bifocals, such as glasses that devote a larger portion of the lens to one or the other Rx, or a variation where the near vision component is at the top of the lenses instead of the bottom.
If you have your various sets of numbers, know what they mean, and can plug them into an order form sure, you can cook up your own Rx… but how thoroughly do you know your own script and what the various parts of it mean?
Now, if you have near and distance numbers and want to somehow extrapolate a middle range… not entirely sure you can do that on your own. You could try it and get a cheap pair on line to test your conclusions. This may require some trial and error, and may or may not cost more than a doctor’s eye exam. Or you could go out and get an Rx from someone who actually has some expertise in these things.
It’s the difference between doing your own plumbing and hiring a plumber. Some things any idiot can manage, other things do require a professional.