Coral growth is a function of 5 things, light, food, pH, Alk and temp. NO, not calcium levels as long as the are above say 375 ppm.
Now, do NOT take this and run with it. There are two ways pH is measured, pHsws and pHnbs. We use pHnbs. So, the true seawater pH when we use a meter is ~0.15 pH units to high. Meaning, if the meter reads 8.3 the real seawater pH is 8.15. About 8.0 is the avg true pH of the coral reefs not 8.3. So, going with a pHnbs as you asked of 8.4, = 8.25sws is higher than any reef. Whether or not it is better to run it this high is really not known, as what takes place in coral growth is depends on those other 4 variables too. It has been shown the Alk is very important growth parameter, much more so than pH. So, running your dKH 1 unit higher would more than likely be a better bet vs that higher pH of your 8.4. And do not ever let that Alk get higher than 11 dKH,i.e., 12 dKH can start issues like RTN. The Alk of most reef is ~ 6.3 dKH or no higher than 6.5 dKH. The reason we often tell you levels like say 9 or 10 dKh is to give you more buffer, not for corals / say but to neutralize acids and help buffer the pH so corals have that reef like dKH.
When we get into the world of pH test kits it is a different matter than a pH probe and depends on the pH test kit color indicator used. Some will give the value like pHsws, others like pHnbs and yet others in between or worse error correction. Meters and kits are affected by the salt. In the pH world it is called the " salt effect error ". And 95 % of the test kit manufactures and 100 % of the aquarium world manufactures do not tell what that error is in seawater. This why people complain about kits vs meters being off from one another, although not knowing the real reason. For example, the SeaTest/Aquarium Sytsems/Instant Oceans/Kordon gives true seawater pHsws using Metacresol Purple. The LaMotte, Hi-Low pH kit uses Cresol Red and when testing in seawater you have to subtract 0.27 from the reading. It is pretty much Metacresol Purple only that gives a good accurate true seawater pH. It is also the choice of chemical oceanographers in labs.
