suppliments are always best added in the AM. It helps prevent large overall daily swings to the pH. Considering pH is at it's lowest point by then, there is less impact.
You'll find it much easier to get the Ca
increase you need if you let the alk
fall off a bit. Stop adding the buffer and just add the Ca
can be added in parts several times a day without much concern as long as they're not too large at one given time.
Personally I'd let the alk
come down to somewhere near 8-8.5 DKH
while the Ca
increases to 415-420 ppm
. Keep track of the alk
and when it reach's the lower point, simpley add enough to maintain it not increase it. For maintenance, use smaller daily doses rather than larger correction doses. It will keep the chemistry much more stable rather than yo-yoing with the larger additions on each side.