IF I score 85% in plus two, how much percentage i need to score in keam engineering entrance inorder to get admission in merit.

A candidate from general category needs to score just 10 marks to be qualified for KEAM. Since you have an excellent percentage in Plus two, it will greatly help in normalization. If you get 700+  marls in KEAM (out of 960) then for sure you will get admission in College of Engineering, Trivandrum and Government engineering College, Thrissur. If you score between 500-700, you'll get admission in any ther government colleges like GEC, Kozhikode; TKM, Kollam; Rajiv Gandhi Institute, Kottayam.

Getting admission to a good engineering college is easy in Kerala. I gave minimal efforts yet I got into a government-controlled engineering college, so I still imagne that if I gave more efforts, then I could have gotten a better college.  Please don't try to score only some set amount of marks, try your best and it is for your own good that you get more marks.