Have you done any official guide questions yet at all? I only ask because it is important to know what to expect on the actual GMAT. Especially on the verbal section it is upon you to know when you begin a question whether this question follows the form that you would see on the actual test.
Having written many questions myself I can tell you how difficult it is to get the formula just right. Basically the official questions must be completely fair - there can only be one answer that is at all correct - and they must also be written in such a way that people are tempted by answers which in hindsight are clearly wrong. This is hard to do! Unofficial questions often stray into the area of more than one answer being at least partially correct (this is the flaw that I am sometimes guilty of) and not having any correct answers (this is the flaw that I see in a large percentage of questions discussed in the verbal section). And of course so many unofficial questions are just not of the same quality as those on the test. Remember, a great question is not written, it is rewritten through many rounds of editing...
I recommend that you do some official guide questions now before completing all of the BTG practice questions. Official GMAT questions cost and average of $2800 each to write according to Business Week and they are the standard by which everything should be judged. Not that all questions should look just like those on the Official Guide but rather you should get a feel for what a real GMAT question is like before you get confused and think that unofficial questions are the standard.
For example, on sentence correction many unofficial questions rely WAY to much on idioms and other tricks. Official GMAT questions are so focused on logic that it really can surprise people. What about Reading Comp? You will notice a certain similarity among all official passages regardless of the topic. You should be able to tell when you are reading a passage if it is appropriate.
On the quant side - unofficial questions rely too much on big scary problem set ups with lots of concepts flying around. Whereas Official questions are known for the following: they guide you to and through the wrong answers so that those people who miss the question are often more confident in their answer than are those who get it right! It is a really smooth trick..and difficult to pull off.
Anyway, I agree to emphasize quality of quantity and with only 40 days left you cannot afford to study anything that does not walk, talk, and smell like the GMAT. So do not save all of those Official questions for the end, there is also the danger that you will not complete them having spent your time on "other" questions.
