Use the CG calculator in the Aerodyanamic Calculators and Online Resources sticky thread. Just input the front wing sizes as the main wing and the rear wing sizes as the stabilizer. It'll give you the CG location you need. If you're not the trusting sort then make up a small 12 inch scale model of your tandem wing design and put the CG in the same scaled spot and try some test gliding to check that it shows a proper degree of positive pitch stability.