There's no need to sweat the degree of polish on the stone too much.
I lap my Jnats to 600x. I stopped lapping past that mark that a good while ago.
I've left more than a few stones lapped to 325x without issue also.
Usually, the stones will break in from use to a much higher degree than you'll get with any type of sandpaper, and that happens fairly quickly.
Most importantly, I've found, is that the surface is continuous across the top - not spotted with glossy and matte areas.
After 600x, I usually use a large Tomo to clear the possibility of errant particles from the sandpaper being stuck in the stone.
I lap my Jnats to 600x. I stopped lapping past that mark that a good while ago.
I've left more than a few stones lapped to 325x without issue also.
Usually, the stones will break in from use to a much higher degree than you'll get with any type of sandpaper, and that happens fairly quickly.
Most importantly, I've found, is that the surface is continuous across the top - not spotted with glossy and matte areas.
After 600x, I usually use a large Tomo to clear the possibility of errant particles from the sandpaper being stuck in the stone.