GMATPrep software is the best indication tool that you can have. There will always be some people who feel it is easier/harder than the real thing -- but who cares? The only thing you should care about is what you have control over. And during practice, you have control over your preparation.
You're really good at math rate questions -- but you stumble at mixture ones. The real thing shows up and you get a difficult mixture question rather than a difficult rate question. Tough luck.
Do what you can now during prep so that you can be as prepared as possible. What actually happens on test day involves a small amount of randomness, but you should be so prepared that none of that even matters.
We encourage you to test out your GMAT skills in our open online Practice Pill Platform:
www.gmatpill.com/gmat-practice-testGMAT PILL video explanations are available for paid members -- along with core course material.