This might be what you're looking for:
https://gmatclub.com/gmat-score-calculator/
That said, it's ALL conjecture. The closest scores to your ACTUAL GMAT score are the GMATPrep. I think it's safe to "underestimate" your MGMAT and VeritasPrep scores by at least 20 points (if not 30-40!) to be safe.
For example, if you got a 680+ consistently on the VeritasPrep CATs, you'd probably get at least a 650 on the actual exam. This isn't based on anything but my own theory, since in general students tend to lose points on the actual GMAT due to nerves.
If I were you, I wouldn't feel REALLY confident going into the GMAT until I scored 40-50 points higher than my do-or-die target score on at least 2-3 practice CATs from MULTIPLE SOURCES (MGMAT, Veritas, GMATPrep).