As mentioned in this post, the first step of your GMAT Prep begins with taking a Diagnostic Test. This need not be a computer adaptive test, the paper-based Diagnostic Test on the OG will suffice.

A few rules that need to be followed while taking the test:
• Take the test when you are fresh so that you can get the most accurate measure of your competence; you should not have anything other than your own ability to attribute a sub-par performance to.
• The test can be taken either as two sections – Quant and Verbal with any length of break in between (even a day or two) – or as five question-types (PS, DS, RC, CR & SC).
• Do not break the test either in the middle of a section or in the middle of a question-type.

Your performance across five categories will be categorized as shown in the table below.

Before we approximate this performance to a score on the 800-scale we have to take a few things into consideration that make this Diagnostic Test easier than the computer-based GMAT:

• It is of a shorter duration — about an hour less.
• It is not adaptive, giving you the freedom to go back and forth between questions, something that is not possible on computer-based test.
• It is paper-based test, making it less cumbersome than viewing on a screen and using scratch pad to solve and calculate or even reading passages off a screen.

Given these reasons, the GMAC itself has the following things to say about the Diagnostic Test:

The Diagnostic Test is designed to give you guidance on how to prepare for the GMAT exam; however, a strong score on one type of question does not guarantee that you will perform as well on the real GMAT exam. The statistical reliability of scores on the Diagnostic Test ranges from 0.75 to 0.89, and the subscale classification is about 85%–90% accurate, meaning that your scores on the Diagnostic Test are a good, but not perfect, measure of how you are likely to perform on the real test.

So factoring in all these things, I have prepared a conversion chart that scales down the performance on this test to account for the relatively easy nature of this test in such a way that even if you answer every single question on the DT correctly, it will equate only to a 750.

The table below will give you an estimate of your scaled scores on Quant & Verbal separately based on the number of questions you answer correctly.

You can use the scaled scores you arrive at using the table above to calculate your total score based on the charts given in this post.

This should give you a reasonably accurate estimation of your score on the 200-800 scale.