I don't know if this is helpful to you, but maybe, so...
I recently ground a broken Parker 45 nib into a stub and used one of those diamond knife sharpening gizmos to give it an initial shape. The one I used was from Smith's, similar to this one: https://www.smithspr...ation-sharpener
Here's the story, with some pictures, in case you're interested: http://www.fountainp...rind-and-rescue
If the specifications on Smith's web site apply to my sharpener, the grit I used for that shaping stage was 750. I would give the nib a few passes, check with a loupe, and repeat as necessary. Checking every few passes with such a low grit is imperative, as the difference from one step to the next is huge (HUGE, I telll you).
Once I had the nib shaped as I wanted, I switched to pads in order to polish and round the corners further.
I didn't use stones because it was just an experiment and I didn't want to spend a lot of money, so I ordered the pads instead.
This article (in Spanish, but nothing google translate couldn't handle) might come handy for you: https://www.apedrada...r-jose-garrido/
The stones used by the author are these ones (80 Euros for a set): https://www.apedrada...ategory=5494243
I *imagine* that the pads are more forgiving because they're not hard, like the stones, and as the nib sinks into them, even if just a minuscule fraction of a millimeter, they "help" you, whereas the stones are hard, which makes them unforgiving if you don't move the pen the right way as you grind.
Note: If memory serves, I watched a video about knife sharpening recently, and it stated that two stones with the same grit may be quite different from one-another when it comes to how aggressively they remove material. I wish I could remember, because I would like to inquire about this. If this is true, grit alone doesn't tell the whole story when it comes to picking a stone for your application.
Good luck and post your results!!!