Hi T, companies love to do this (ask for your salary requirements) because they are counting on you being nervous about getting the position and are trying to get you to price yourself too low (something women are especially likely to do). So, I have some advice for you.
First, be absolutely sure about what your bottom line is. If it is x, make sure you feel that if they can't get as high as x that you will feel comfortable walking away. Of course, you never reveal x to the other side but it is helpful for your decision-making.
Second, assess y, your target or desired amount.
Third, figure out a range for your salary expectation. I always tell people to give a range, make it kind of large, and to add a note with it. So, once you have x and y, you would say that your range is x+e to y+c where e is some small amount and c is a larger amount. So, say your bottom line is 60,000 and your desire is 75,000. Say your salary requirement is 65,000 to 90,000. And add on the form that you need to learn more about this position to determine your exact salary requirements.
A couple of other things from your posting. First, why is your "desire" so much less than you think the position should earn? This signals to me that your desire or target is probably not high enough. Second, you note that it might be difficult to use published sources to identify a fair salary for the position. In that case, ask your friends and professional contacts what they think the position is worth (so you get around asking them the sticky question of what THEY earn). Third, try to avoid negotiating until they've offered you the job. You'll be in a much stronger negotiating position once they've decided they want you!