I might be a little late on this topic but I keep my GEO reactor at 6.69-6.70 and this is how I was instructed on how to adjust them correctly.
Before setting up the reactor, you should check the alk and calc levels first and make sure they are at the proper levels. Since alk and calc have pretty definded relation to each other, you will only need to check the alk in the setup process. This is how to find the sweet spot of the reactor you are using.
Once you get a Idea of where the alk levels are sitting, you wait 24hrs and check the alk level again. Is the level decreasing? if not check the level in another 24hrs. (If the levels are still not decreasing, check again after 24hrs and you may wont to reconsider if you really need a reactor for your tank at this point)
If the alk level has begun to drop, start your calc reactor with an effluent flow of about 50ml per minute and an internal ph level of 7.0. Let the reactor run at this level for 24 hours. After 24hrs check the alk again. If the level is still dropping reduce the internal ph to 6.8 or 6.9. Again check this process in 24hrs to see if the alk level has stabalized. Once you hit the "sweet spot" the alk level in the tank will remain steady.
Now if you are checking the alk and you reach 6.5ph or lower in your reactor, reduce the flow amount of the effluent to 45-40ml per minute, and start the process again at 7.0ph internally. continue to check the alk levels every 24 hours while making adjustments until your alk stabalizes.
When you finally get to the point where the alk levels are steady, you can now do a one time dosing of the tank to get the alk and calc levels back where you would like them to stay. This is something that will need to be checked every few months due to tank growth or addition of large amounts of coral.
That is the basics for everyone, if you have any question hopefully I can help