First... I would describe the use you are proposing as fertilizing the garden, not ammending. Ammendments improve soil structure and balance. Synthetic fertilizers do not do anything to aide structure, they strictly look at short term nutrient availability. As you noted, they can do harm if improperly used.
I have used 10-10-10 many times over the years (prior to installing the raised beds). I can not locate it now but I once read an ag extension document that stated something to this effect....
If you have a garden soil of unknown fertility it was safe to apply 1 to 2 lb of 10-10-10 per 100 sq ft in a broadcast application and incorporate in the top several inches of soil. I did that 1 lb for many years when I flat gardened with productive result. I also followed up with side-dress applications or banding applications.
You might want to go through a number of ag extension sites for guidance on using the chemical ferts. You can find info in just about every state and you don't have to limit your reading to yoour own...
http://ohioline.osu.edu/hyg-fact/1000/1601.html
http://pubs.ext.vt.edu/426/426-323/426-323.html