I don't know how you can recalibrate a treadmill.  I use an elliptical trainer sometimes and I ignore the calorie counter on there - it tends to overestimate.  I estimate my calories burned compared to the "one mile = 100 calories" formula.  That is also just a rule of thumb but it is good for a general estimate.  I know it takes me about 10-11 minutes to run a mile (or burn 100 calories) at a moderate intensity pace.  So if I am on the elliptical, which is easier than running, I translate that into about 12-15 minutes = 100 calories depending on my perceived level of exertion.  I have been running long enough to know if I am working out at an intensity that is about equal to a moderate running pace.  Also, I can use my HR to double check that estimate since I have an idea what my HR is when I'm running that pace.

From what I understand a treadmill is also easier than running outside so you could do something similiar.   Add a couple of minutes to the time it takes you to run a mile outside and call it 100 calories (more or less.)