I use Salifert for all of my testing as well. Although most of them are fairly tolerant of slight testing inconsistencies, the Mg test really needs to be done exactly how the instructions state.
after adding 5 drops of mg1, swirl for 30 seconds. then add 1 spoon of mg2, swirl for 10 seconds. when adding mg3, with the other tests you can get away with adding about 1/2 a syringe then drop at a time until color change. For the Mg test, I get the most consistent results adding one drop at a time from the beginning until color change.
I use the stopwatch on my phone to time my "swirls".
Using this method I can get very reliable, repeatable results. Like mentioned above, the test kits will not give you laboratory level precision, but they will be consistent and that is really what you need to be concerned with IMO.