Good question, and I'd say: yes, taking a mock at 40% syllabus is totally fine — but go in with the right expectations.
Here's how I thought about it during my own prep. A mock at this stage isn't a score predictor — it's a diagnostic. You haven't seen all the concepts yet, so you WILL struggle in areas you haven't studied. That's the whole point. The value is in understanding which untested areas hurt you most, so you can prioritize them in the next phase.
A few concrete guidelines that worked well for me:
Don't wait for "100% syllabus complete" to take your first full mock. That day often never comes, and you miss weeks of data about your pacing, stamina, and weak spots. I took my first GMAT Focus Official Practice Exam after about 40-50% of my quant prep, and the result was humbling but extremely useful.
Take the official mocks (GMAT Focus Edition has 6 official practice exams, 2 free). Save at least 4 of them for the final 4-6 weeks before your test date. Use third-party mocks (like
GMAT Club tests, GMATPrep,
Magoosh) earlier in your prep to avoid burning the official ones.
After each mock, spend 2-3x the test time reviewing. Every wrong answer and every question you got right by guessing is a learning opportunity. Categorize errors: concept gaps vs. careless mistakes vs. timing issues. These need different fixes.
You're asking the right question at the right time. Keep studying hard and use each mock as a mirror, not a verdict.