If you trust the recommended developing time you have for EI 250 I suggest that you use that as a starting point for your EI 200 roll.
Regarding determination of developing times for different EI (Exposure Index). Hidden in these two recent threads are some good pointers, as well as descriptions of different approaches:
However, I do believe that reading a well thought through published text on the subject is a much better alternative.
Try one of the following books, several of them should be in print and it is quite possible that you will find some of them at a library:
Les McLean – Creative black and white photography
John P. Schaefer – Basic techniques of photography, Book I and Book II (Book II has the most detailed description).
Ansel Adams – The Negative
Even if I have not read the following two books I believe that I can recommend Edge of Darkness, by Barry Thornton and Beyond The Zone System by Phil Davis.