I found an explanation on converting the number of drops of the API kit to degrees and ppm. One drop = one German degree and if I multiply the drops by 17.9 I get the ppm.
My KH and GH tests on my well water both took 26 drops to change color. I also had it checked by a local fish club member, and by the local fish specialty store with similar results.
So, KH and GH would be 26 degrees hardness, or 465.4 ppm
My PH is at the highest range of the "lower" PH test in the Freshwater Master kit - and if I use the High Range PH test, it is hard to tell, but somewhere between 7.8 and 8.2. The local fish club guy tested it with a better test kit and said 8.2 to 8.4
Not much can live in that. Knowing how hard the water is, and how alkaline, kept me from even trying an aquarium for nearly 30 years. I'm really not into livebearers or brackish type fish, although I did briefly consider a puffer.
I've got a betta in a 5 gal Fluval Spec V that seems to be doing fine, but the ends of his fins look funny - someone on a betta breeding forum told me, before I even got the betta - that the fin edges can be affected by high PH, but that it won't really affect the health of the betta.
The other fish, shrimp, snails are in a 15 gallon.
I am having issues with a lot of the plants too - the fish club person gave me at least 20 different varieties to try and see what would live. While many are still living - few are thriving.
So, if I use the 2/3 RO to 1/3 well water, I get
PH 7.2 to 7.4
GH 10.3 degrees or 185 ppm
KH 7.9 degrees or 143 ppm
I thought I should mix up more of it, leave it sit for a week or two, and test a few times to see if all the numbers remain stable.
It is my understanding that using a mix of RO and well water will be easier to get to remain stable than using RO and adding stuff to it.
I did purchase PH Down when I bought the Fluval - aquarium store guy said I would need it - but after looking it up I have not used it. The info I found said similar to your experience, that it is only temporarily down.... and my understanding is that we want it to be stable no matter the number.
I'm sure it would be better to go with the well water as is, then find fish and plants that can work in it - but from all the reading I have done - there is not a lot of choice with how hard my water is and the high PH.