Hi,
I believe by 'diagnostic' test you mean GMAT Prep from mba.com. As far as the strategy for GMAT prep is concerned, you are fine. However, I would like to add that since there are only two test available, so you can again reset the tests. You may find some questions from previous tests but at the same time you will find many new questions too. The question type of GMAT Prep is very near to the real GMAT and for that reason I would advice GMAT prep.
Now coming back to your question, how many other test one needs to give. Well, the answer is very subjective. Depending upon your grey areas,areas where you need to work. Preferably 4-5 CAT's would help you to get better acquainted with the TEST.
There is something I want to share from my experience. Why don't you make your study a little interesting. Look for the areas you are weak. Lets say, you are weak in DS, so instead of solving those DS questions vaguely, try solving them in the form of test. Put a time constraint and keep a track of your errors. So in that way you will be able to gather stats about your performance as well as you will be able to enjoy your studies.
So in that way you can create your own tests!!
