What is the difference in cook times of a 1100 watt Microwave and a 700 watt Microwave?

You could use this equation:

(1100W/1200W) * 6min = 5.5min

However, microwaves vary considerably and just using wattage may not give the correct result. Unfortunately, trial and error is probably the only way to get the correct time. Additionally, many factors will change the cooking times such as the container that the food is in, and where in the microwave the food is placed. The best method would be to use a food thermometer to measure the temperature of the food, and change the cooking time to get to the correct temperature.