Just so we're on the same page...you are talking about 2 seperate things. pH and calcium.
A pH of 7.5 is low while 8.0 is fine. When are you testing it? It's normal for a tank to have a lower pH in the AM before the lights come on and a higher pH in the evening after the lights have been on all day. Using any buffer is not a good way to try to control pH. The stuff you are using is baking soda and boron. The boron will temporarily hold the pH steady, but can also raise the tanks borate level to potentially dangerous levels. The baking soda will raise the tanks alkalinity. The more you add, the higher it goes. What is your current alk
Here are a few articles you can read.
Low pH: Causes and Cures by Randy Holmes-Farley - Reefkeeping.com
Boron in a Reef Tank by Randy Holmes-Farley
Your calcium is fine at 400-420. What salt do you use? How often do you do water changes and how big are they? As far as figuring out how much you need to dose or if you need to dose at all, you must test your water. Test it today. If it's at 420, how many days does it take to drop to 380? Also keep in mind many test kits only measure calcium in increments of 20, so you can't be sure the color turn happened at 19 drops or 20 drops...or would a 1/2 drop have made the change? Bottom line is test, test and test again before dosing.
This is an excellent "must read" article.
The “How To” Guide to Reef Aquarium Chemistry for Beginners, Part 2: What Chemicals Must be Supplemented