Hi variantguy,
I’m glad you reached out, and I’m happy to help. First off, 730 is a nice practice test score, especially because you left the last quant and verbal question blank. Regarding that issue, to ensure that you do not make the same mistake again, you need to be more disciplined, so you are answering the last question with at least 15 seconds left on the clock. In that case, I do not envision a scenario in which any answer choices will be left blank.
Regarding how to improve by 20 points to reach 750, you’ll have to spend some more time finding and fixing your remaining quant and verbal weaknesses. To do so, in addition to taking practice exams, you may consider engaging in some focused practice in both quant and verbal. For instance, let’s say you are practicing Number Properties; answer 50 or more questions just from Number Properties. When you do dozens of questions of the same type one after the other, you learn just what it takes to get questions of that type correct consistently. If you aren't getting close to 90 percent of questions of a certain type correct, go back and seek to better understand how that type of question works, and then do more questions of that type until you get to around at least 90 percent accuracy in your training. If you get 100 percent of some sets correct, even better. Number Properties is just one example; follow this process for all quant and verbal topics.
As you do such practice, do a thorough analysis of each question that you don't get right. If you got a remainder question wrong, ask yourself why. Did you make a careless mistake? Did you not properly apply the remainder formula? Was there a concept you did not understand in the question? By carefully analyzing your mistakes, you will be able to efficiently fix your weaknesses and in turn improve your GMAT quant and verbal skills.
Lastly, as many have stated, if for some reason things don’t go your way on your upcoming exam, you can always study some more and retake once you are ready.
If you have any further questions, feel free to reach out. Good luck!