model based structure refinement

[muhammad ali]

dear all
if the sufficient knowledge about the structure is known(eg. lattice 
parameters, space group, approximate atomic positions etc), then what must be 
the stategy to solve similar structures.

i m working on soft ferrites which have spinel structureand space group Fd-3m.

i have made a lot of literature survey, but i cannot find the proper guidance 
about it

thanks in anticipation