Don't worry too much about which mic element to use - I'll try and calibrate mine this weekend but I expect to find it's flatish from about 100Hz to 10KHz and just droops a bit outside this range. Pretty much any small condenser element should behave similarly.

Maybe someone over there in the US could choose an easily obtainable element and calibrate it (or you could post a sample to me and I'll do it, but my setup isn't exactly professional ).

One with a published curve would be better of course.

Cheers, John