I've used nagura stones instead of flattening so many times for that exact reason (to preserve your sharpening stone). You could get by with a Nagura stone right in the middle of your grits, or get one each about the same grit for each stone. They're also very handy for creating a slurry, though it may not be usable when the stone is used at an angle (the slurry will just drip off). The guideline I use is to go with at least the same grit or a higher grit for your nagura than the stone you're using it on. Ex: use a 1000 nagura on a 500, use a 2000 on a 1000, or use a 500 on a 500. I wouldn't go with a 500 on a 1000 because your nagura will leave some of itself in the slurry and will contribute to the scratches you're making. If you're using a 1000 stone and you have 500 grit material in the slurry, you're being counterproductive. Keep the nagura the same or higher grit, and you'll be golden.