Interesting about the discordia test result. Lots of ISTPs seem to initially test as INTJ.
Look here for a small overview:
ISTP - INTP/INTJ
A really good way to figure out your type is to look through these descriptions and find out which suits you most in terms of content and style.
ISTJ
INTJ
ISTP (Keep in mind that 'J' does not necessarily mean what you think it means.)