The aggregate of the 12th boards does help boost your score up to a certain degree. 170 in a KCET is a very good score and you have a really good chance at getting into a good college with a good subject.
To qualify KCET, you have to fulfill the minimum requirements of the 12th board. You have to have passed your 12th standard examination or it's equivalent in any other board. For the general category, a minimum of 45% marks is required, and 40% for the reserved category.
In your case, if you have scored less in your 12th boards, it would slightly affect your ranking with the candidates that have scored the same as you on the KCET. Nevertheless, 80% is a good score in the 12th examinations. You can always refer to the ranking system of the previous years to estimate your possible rank to the nearest 100.