I think the very first you need to know is the market. Talk with colleagues, better if they recently moved, and ask for a little more. I think most of my friends know my salary, some earn more and some less, who cares? I'm not sure about not mentioning your current salary because a lot of interviews have ended at that point without wasting anyone's time. You can give a figure and ask for a 30% raise, it's perfectly reasonable because moving has always some associated risk and HR knows, better than anyone, that it is your better chance at improving your salary.