Theoretical break frequency and bandwidth of notch filter
There is an excellent on-line calculator here which contains all the formulas for a twin T notch filter - your circuit isn't a twin T but you can use the formulas as shown below: -



Firstly R3 can be assumed to be infinite and secondly C2 and C3 can be both assumed to be 200nF thus making 100nF in total.

I reckon your filter is notching at 250Hz (according to the online calculator). Good luck with calculating the bandwidth from the formula. 

EDIT Here is a start for you: -