One can find a good base value for the offset with simple math.
Look at Shockwave's missile speed. It's 1050. That means it moves 1050 distance units per second. Then a trigger that runs once every 0.03 seconds runs 1/0.03= 33,333... times per second So the offset (=speed) would be good to set to 1050/(1/0.03)=31,5.
This does not always make the unit travel in a straight line. I for example wanted to the "shockwave" dummy to do damage to the target when they're closer than 16 distance units from eachother. It didn't always fire since they never got that close because the dummy didn't travel in a straight line. I moved on to use a periodic offset trigger, and it started working.