Your GMAT score depends on a lot more than the percent right or wrong. The level of difficulty and where on the test you got your wrong answers determines your score much more than the raw number of right vs. wrong does.

That said, to get a 450, you should be getting most of the easy questions right and roughly half of the medium level questions right. You can use the Official Guide as an excellent indicator, since the questions move in order of difficulty.

Also, you should take a practice test to determine your baseline - here's a free one from Veritas.

https://www.veritasprep.com/gmat-prepara ... ctice-GMAT

Good luck!