Hi Raghav21oct,
When it comes to properly assessing your overall GMAT skills during your studies, there is really only one way to do that: by taking a FULL-LENGTH CAT under realistic testing conditions (meaning that you must take the FULL CAT - including the Essay and IR sections, take it at the same time that you plan to take your Official GMAT, take it away from your home if possible, etc.). While a "mini-test" might be useful in pointing out content areas that you need to work on, there would be no practical way to assess what that data would mean on a scoring scale out of 800, so taking one of those shorter tests is not the best way to initially measure your skills.
The scoring algorithm on the Official GMAT is far more complicated than most people realize. Since that algorithm is proprietary, no GMAT company has an exact match for it, thus CAT scores can vary a bit based on the 'biases' involved in their respective designs. The most realistic CATs available are the 6 from GMAC, but the CATs from Kaplan,
MGMAT and Veritas are all 'close enough' to the real thing that they will provide you with a relatively realistic score assessment (assuming that you correctly take the CAT in a realistic fashion).
GMAT assassins aren't born, they're made,
Rich