You cannot relate your Mock test score to the original exam score. Mock tests are designed to be tougher than the original exam to help you prepare. But if you have scored 623, it means that ideally your basics are good. And if you can apply the same basics and concepts to NEET, you can secure more than 600. Practise as many mock tests as you can that are available online and stick to the time. Make sure you are able to complete all 180 questions on time. Also memorise formulae and theories once again. This should help you with a score more than 600. And this score will help you with a seat in a Government Medical College irrespective of your Category. This year as NEET has been pushed by 5 months, preparation time has been increased and so competition is going to be tougher. And so the chances of higher cut offs are more.