When I used the ascorbic acid I didn't dilute it in water. I just poured the ascorbic acid down the walls of the pool, concentrate more where you see stain. You will probably need less than a pound. You can shut the filter off, or leave it on - the most important thing is to get a sequestering agent in the water, so that when the stain lifts off the surface of the pool and goes into solution, the sequesterant will hold it in suspension. After an ascorbic acid treatment you DO NOT shock your pool for 2 weeks.
1. Pour ascorbic acid down the sides of pool
2. Let water circulate for 1/2 hour
3. Pour in enough sequestering agent for the gal size of your pool (I would use a bottle of something like metal free, sequasol, or Jacks Magic.
4. Let the filter run 24/7
5. The next day check ph, add borax to take it back up to 7. to 7.2 (no higher)
6. After 48 hours start to raise chlorine slowly. It will be hard to get the water to hold chlorine because the chlorine will eat up the ascorbic acid that is in the water.
Your alkalinity is a little high, but the ascorbic acid may take it down. Make sure you check it also. If it is too low, you can add baking soda to bring your ph and alk up. Add small amounts and check at 2 hour intervals. With a cya of 50, you can take your chlorine up to 6, but not higher for 2 weeks. If you see stain start to reappear, then take your ph back down to 7 and add more sequesterant. Keep me informed, and feel free to ask any questions you may have.