How accurate is GMAT Mock 1 from Mba.com?

Those official tests are, by far, the most reliable assessment tool you have available, and are essentially identical to the real test. If you knew some questions in advance, then that may have inflated your score, unfortunately. That's one reason to try to avoid seeing GMATPrep questions before using those tests (which is sometimes hard to do on gmatclub unless you pay careful attention to source tags). If you do see a familiar question on a diagnostic test, you can prevent it from having too much of an effect on your score if you do the following:

- answer correctly if you honestly think you'd answer correctly if you'd never seen the problem before
- guess randomly, using a random number generator of some kind, if you think you'd need to guess if you'd never seen the question before (you'll be penalizing yourself too much if you intentionally answer incorrectly, something you'd of course never do on a real test)
- let a realistic amount of time elapse -- do any calculations or reading, for example, that you'd do if you had never seen the question before, and let some additional time go by to account for any thinking you'd need to do

Then a small number of familiar questions won't really affect your score, but if you know too many questions, then a test may not end up being usable. Then the best tests you could use would be the extra official ones that you can purchase, though I'd be careful about when you choose to use them, since you don't have very many. There's not much reason to do more than one test early on in your prep, and then you're doing that just to figure out how long you might need to study, and to get familiar with what the real GMAT is like, so you can ignore prep materials that aren't realistic or aren't teaching the right things.

Even with a few familiar questions, a Q49/V40 is an excellent score, and I'd imagine you're not far from that level, though an unfamiliar test will give you more reliable information. Good luck!