Add the salt first, then add the buffer if needed. Do not add buffer for 24hrs after mixing the salt, then test and add if necessary. To determine what your alk should be, you'll have to know what the CA is.
I have both the Seatest and the Deep Six. I used them both and rinsed them in fresh water after each use. When I tested them against each other I started to notice differences. If I ran several tests in a row I also noticed some differences. I ended up getting a refractometer ($50 at diyreef). I trust that but still check it weekly, both against RO/DI (0ppt) and against the supplied calibration solution (30ppt).