we're tryin to help...If your alkalinity is at 3.8meq/L and your calcium is at, say, 420ppm, your water will be fine and in balance. Don't be afraid to use those links that I posted for you...they are a big help. The first of the two links....
http://www.reefnerds.com/ChemPlot/ChemPlot.html
makes it very easy to see what calcium levels will be compatable with an alkalinity of 3.8. You just have to plug in the numbers.
...and to answer your question directly...it'll be fine to add the Seachem Reef Buffer but be sure to follow the instructions on the container that it comes in along with testing for pH and alkalinity while you do apply it.