They can't predict with 100% accuracy, because vision isn't a completely objective matter as it also takes into account your brain's interpretation of the image, but they can get pretty close. The exams you took probably measured your eye's axial length, your cornea's keratometry, diameter and other measurements.
Your ophthalmologist then selects the formula that best suits your eye (there's different mathematical models for different cases of myopia, hypermetropia and how extreme they are) and then the lens' power is calculated according to the measurements that were taken. Usually the device that takes your exam already does like 80% of the job (in the mathematical side of things), but your doctor uses their criteria to define the final IOL and from where it'll be inserted during surgery*.
It's pretty cool to take that exam. In my country I used to take it for patients that were going into eye surgery.
* It usually means a little bit more math
I'm not sure if there's something as healthy cataracts. Every case of cataracts has to be eventually treated by removal of your eye's lens and then replace it with an intraocular lens (IOL).
If you meant if its ever done on a healthy lens, I'm not really sure. There's a technique for installing an IOL on top of a previously installed IOL that's called piggyback IOL, but on a healthy lens without cataracts seems uncommon.
Lensectomy and IOL placement can occur when the patient has a healthy lens in some cases though. When they're going through other issues such as proliferative diabetic retinopathy, fibrovascular proliferation can occur in the vitreous humor, which would require removing it (the procedure is called vitrectomy). In some of those cases, the replacement of the vitreous humor accelerates cataracts development, which means that the lens will have to be replaced with an IOL.
That's what comes to mind from my experience doing those exams many years ago, I could be wrong.