Before you make any more changes to your water chemistry regarding TA and pH, it would probably be best to look at the your potential for scaling or corrosion right now, based on your numbers.
Have you found the Pool Calculator in Pool School? There is a link to it in my sig line.
Now, I don't have your water temperature, but by plugging in your numbers in the Pool Calculator and giving your water temp an estimate of 70 degrees, I am getting a fairly negative CSI reading which indicates that corrosion to surfaces, not scale buildup, to be your most pressing potential problem. These numbers (and the temperature estimate) give me a CSI of -0.53. This is very close to being a problem for your plaster and surfaces that come in contact with the water.
I would also raise the FC. Based on your CYA of 70, 5 ppm FC is the bare minimum. It should be closer to 8 ppm to be ideal.
As far as the ring around your water line, if it is related to calcium, you can try a stiff brush and a VERY dilute muriatic acid / water mixture and see if that will help. Remember though, anytime you apply an acidic material on your rocks or grout, you risk taking some material that you don't want to remove off.