I am an ISTJ - does that map to a S or C in Disc? Or should I just sit down and do the 90 Disc question survey?
There isn’t a great deal of scholarly work showing the way these two assessments relate to one another. The DiSC manual itself suggests that only the i scale has any significant direct relationship with the I/E scale from MBTI. Even then it isn’t a very strong relationship really. A deeper factor analysis of the original items would probably show an entirely different structure inherent in both assessments when combined and you could probably work out some conversions but it would be rather different from either original assessment models or scales so would be hard to interpret within their frameworks.
I think you need to do the DiSC if you want to use their materials to attempt to understand yourself
Here is a link to the manual too - page 11-12 has the relationship statistics.
Definitely a C in my opinion! But not definitive
I don’t think there is a official answer to this but as someone who has done hundreds of Disc assessments and hundreds of MBTI assessments, I suspect you will report as a C simply because of your I and T in MBTI.