Hi,
I think you're making this out to be a more complicated operation than it needs to be. I would suggest getting a good book on XRD from the library (I like Cullity). It will nearly walk you through the steps to find a, b, and c.
Now, the book will give you a formula for bragg angle (NOT 2 THETA) as a function of h,k,l and a,b,c. Knowing that the a,b,c d-spacings correspond to the (001) type planes you can find a,b, and c easily. Hope this helps.