Is past years cutoff for csab special round reliable to use as guideline in 2018 csab special round?
See this special round cutoff always varies year to year. But still you can use it as a reference just to get an idea about which college you will be allotted.
Cutoffs do vary each year but past year cutoffs is the best reference available for you to know your chances. So even though not completely reliable, they are useful.