Question 536330
I'm not sure if this is a math problem, or just cooking in general. But from personal experience it is not a good idea to cook the bird at a lower temperature, you risk more infection and less chance of it reaching its proper internal temperature of 165 degrees.
The rules of cooking a turkey go as follows:
Here is an approximate turkey cooking time guide for roasting at 325 degrees F. from the USDA:
8 to 12 lbs: 2 3/4 to 3 hrs
12 to 14 lbs: 3 to 3 3/4 hrs
14 to 18 lbs: 3 3/4 to 4 1/4 hrs
18 to 20 lbs: 4 1/4 to 4 1/2 hrs
20 to 24 lbs: 4 1/2 to 5 hrs 

For 250 degrees, I would double your cooking time..depending on the size of your bird. And I would most definitely check the internal temp before eating. 

I recommend this site for you to read if you want to know more..and Happy Thanksgiving!
http://busycooks.about.com/od/thanksgiving/a/unsafeturkeymet.htm

If this is a Math problem, just send me a little note, and I'll edit my answer:)