Sorry can't give you a direct answer to this, but I do know there's several versions and variations of Quaternion function libraries in the code archives! These libraries all handle the same basic functions (Euler to Quats, Quats to Eulers, Slerps, etc) so I think it's safe to say that you should be able to find a solution in one of there. Either copy one of their functions or fix/adjust your own using theirs to compare...
Hope that helps,
Danny.
Hope that helps,
Danny.