Use equation for microstrip differential Zo equation (provided in the Appendix of AN-905) or use National's Rapidesigner to solve.  If using the Rapidesigner, the three steps are required:

1. In Diff Zo, set ratio of h/s at 20/12.5, read microstrip factor of 0.735
2. Align Zo of 68 to 0.735 factor
3. Read Zdiff of 100 Ohms