Daggerboard placement will depend on what you want to achieve...if you want to give the 'shared lift' concept a shot, put them way forward of the CE. If you want to go with a more traditional line of thought, put them on or slightly behind the CE for more balance with the head sail up.

For the purpose of this generic discussion, you can approximate the Center of Lift from the sails at approximately 1/3 back from the leading edge. You should consider where that center of lift goes with the headsail working too. However, it's hard to do an accurate calculation without going into drastic detail about lift from each sail and the various lift angles in 3D. I suggest you take a design that you like (Taipan, Nacra 5.2, etc.) and perform this generic calculation with sail area and assuming lift at 1/3 back from the leading edge to determine the position of the daggerboards relative to the sail drive. Then use the parameters learned with the parameters of your boat to determine a relative daggerboard placement ... that would help eliminate some of the wild details you would have to accomodate.