I have my first attempt at NMAT on 2nd jan. Today I took NMAT official mock and scored 268 marks(LS/LR/QA : 84/94/90). how realistic is this score with actual NMAT taken into consideration.

NMAT won't take it into consideration, but the score is spot on. I got 277, both in the official mock and the actual test, just the individual section scores were different. So you should just give the attempt as you gave the Mock, and it will surely go well.