Which board and how many grace marks did u got? If you're aggregate pcb is more than 50% then you're eligible don't worry about the grace mark that is given my every board in India except cbse and it is acceptable, reappearing 12th again will make you a private candidate,NMC doesn't want private candidate to appear for neet, every year this case gets hold on supreme court,so it's better if you don't give improvement exam if you already have pcb  50% combination

Grace mark is not a problem you can still appear for neet don't worry