It's impossible to accurately translate that sort of number into a scaled GMAT score, unfortunately. The GMAT gives you a score based on which
questions you miss, and not necessarily on how many you miss, and if you're just doing practice problems, then you're not facing the time pressure that you would have to deal with on an actual adaptive test. Your result sounds pretty good, but it's a completely different task than taking an adaptive exam.
It's also impossible to say how many hard questions you'll see on the test. If you're doing really badly on an adaptive test, you won't see any hard questions. If you're doing really, really well, nearly all of your questions will be incredibly difficult. The GMAT is funny that way.
It's definitely a good idea to do the practice questions from the GMATPrep software and the GMAT official guides, but if you want to know what your score is, take an adaptive practice test. The GMATPrep tests
are the most accurate, but you might want to save those until you're close to your exam date. Unless you want to take a GMATPrep test now, MGMAT is probably the best bet to give you a sense of where you stand.
I hope this helps!
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