It's definitely a percentage. I don't remember how much any more, but used to know for rank 5, which is all I think I've bought. I'll see if I can find more current info and put it in. I liked it. I wouldn't rank it about some other more critical things, but if you tank / get hit alot, it's worthwhile.
EDIT: Other places say specific amount of DS. I'll be adding a DS column and totals to it, shortly.