QA-OB-7: The rules say you can mark relative to where the disc “last crossed into OB”. At what point does that happen? For example, a disc may fly above the OB line for a while. Is that point where part of the disc first crossed the line, or when the entire disc crossed the line?
It’s when the entire disc crossed the line. To be super-technical, since the disc is a circle, there will be a single point of last contact with the inner edge of the OB line. That is the point you use for marking.