Hi Kate.li88,
GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Assuming a similar 'swing' in how your CATs function, your various score results show that you essentially performed the same each time (around 450 +/- a few points; the 540 is an outlier that we can't really assume is an accurate reflection of your current skills).
Using this 420 as a gauge - and the idea that you need at least a 600 to competitively apply to this Business School (and significantly higher to get a Scholarship) - you will likely need at least another 3 months of consistent, guided study to get to that level. Thus, the first application deadline (in late August) does not seem realistic - and the early October deadline would be the better option. If there's another application Round after that, then that should be another option to consider.
1) What order did you take the GMAT sections in?
2) What were your Quant and Verbal Scaled Scores for this 420?
3) Going forward, how many hours do you think you can consistently study each week?
GMAT assassins aren't born, they're made,
Rich