Hi!
I had a similar question a while back prior to taking the GMAT for the second time.
The GMAT is designed in such a way that you will not be able to answer more than 80% of the questions correctly (at the very most - if you're in the 760+ range, most likely). Due to its very nature, the GMAT will keep ramping up the difficulty level until you can no longer answer questions correctly. This does not necessarily mean that your score is actively decreasing with every mistake that you make. Naturally, the
OG only has a small number of 700+ level questions - at the very end of each section (probably the last 20-30 in my humble opinion).
In my
OG-related experience, I had an accuracy rate of approximately 80% in Quant and 90% in Verbal. This ultimately translated into a 49 Q score and a 42 V score for a combined 740.
What you really need to do is focus on ironing out the simplistic mistakes; someone aiming for a 720+, like yourself, should not be prone to making errors on 500-level questions. Once you finish the
OG, seek out 700+ level problems. Since your exam is in May, you still have a good deal of time. The very best gauge of your ability is to use the GMATPrep software to take a mock test. It will tell you how far along you are, and how much further you need to go to achieve your target score.