What ratio of MPC and WPC is best for Greek-style yogurt?

It depends entirely on the desired yogurt protein content. We would recommend:

6.4% Protein Greek-style yogurt: 3.2% protein from the yogurt milk, 3.2% protein from MPC or an MPC/WPC blend. 100% MPC or a 90% MPC/10% WPC blend.

7.0% Protein Greek-style yogurt: 3.2% protein from the yogurt milk, 3.0% milk protein from MPC, and 0.8% protein from WPC. The MPC/WPC blend would be 79% MPC and 21% WPC.

9.5% Protein Greek-style yogurt: 3.2% protein from the yogurt milk, 3.2% milk protein from MPC, and 3.1% protein from WPC. This blend would be 51% MPC and 49% WPC.