First of all the region of ur polytechnic must be selected because it might differ from place to place talking about specifically Gujarat polytechnic colleges when I completed my 10th standard the admission here were done on the basis of marks of English,Maths and science and then we were given ranks on basis of this score . If your total is 250+ then you have high chance of getting govt polytechnic for general . Don't know exactly about other categories. But your minimum must be 230+ then it may fetch u a govt polytechnic.
Delhi Polytechnic (CET Delhi) is having approx. 6000 seats. If you belong to Delhi, your rank must be below 4000 for opting core branch and best college. If you belong outside Delhi, you must have to secure a rank below to 1500.