I’ll be honest: some GMAT practice tests are more accurate than others. The official GMAT prep test from ETS materials is the most accurate. Other practice tests are pretty accurate but could be a little harder or easier. I would use GMAT practice tests as ways to gauge a score range. It’s safe to estimate a small range better or worse. To make your practice tests as accurate as possible, mimic the environment of a test day. Don’t take more breaks than you normally would be given. Don’t give yourself more time. The best way to improve GMAT scores, other than studying, is to practice pacing.