Your question is actually backwards.
You can do it either way, really, but at Spark we do it by stating how much exercise we will do, and then calculating the food. You don't calculate the exercise.
So how much "should" you do? As much as you feel comfortable and safe doing and can see yourself doing for life.
If you're brand new to exercise, the suggestion of 20-30 minutes 3x a week is a great start. See how you feel. If you're capable of doing more, then do more.
Don't worry too much about the goal for one week - just do whatever feels okay to you and track that. At the end of that week you'll have an idea from what you've tracked as to how much exercise you will be able to do weekly. You can plug in that total calories burned as next week's goal.