For 100% modulation, the upper sine wave needs to swing from vcc/2 to vcc, and the lower sine wave needs to swing from vcc/2 to 0V. In order to make this happen, the DC bias on the upper sine wave needs to be 75% of Vcc if you are using rail-to-rail op amps. When you invert the upper sine wave around vcc/2, the lower sine wave will be biased at 25% of vcc, which is what you need.
If you are not using rail-to-rail op amps, tell us what you want to use and we can tell you the bias voltage that will give you the highest possible amplitude 100% modulation without audio distortion or overmodulation.
Below is a simulated output waveform of my last circuit, but with Vcc=+14V and Vin=0.7V p-p (0.35V peak). This is the highest-level 100% modulation waveform you can achieve with a 14V supply. If you make the bias voltage lower, you can still get 100% modulation, but at a lower amplitude. If you make the bias voltage higher, you will not be able to achieve 100% modulation without clipping the peaks of the audio.
The ratio of R3/R4 is calculated from this relationship:
R3/(R3+R4)=0.75