Nexus22,
Just a correction on your sentence:
"How much realistic are gmatclub free tests."
Should be rephrased as:
"How realistic are the GMATClub free tests"
The quant section is generally tougher than practice questions elsewhere -- so if you scored a 51 then you are in pretty good hands. According to this
scoring table, a Q51 and V51 would've gotten you a solid 700 score of 750.
This so far looks like you're in a good zone. However, never underestimate what will actually occur on the real test day. There are many test takers that do well on practice tests and then end up having the nerves on test day and scoring much lower. Practice and study as best as you can -- at gmatpill we recommend using framework strategies to tackle questions.
In the end, the GMAT Prep practice tests are the most "accurate" practice test -- but you only get 2 such practice tests in the software. Just make sure you are able to take one of these GMAT Prep practice tests 1 week before your real test so you have an accurate gauge going in. You can see this
GMAT study schedule as an example of how to time the last GMAT Prep test.