Did you extend the center line after you made the .5mm offset?? ?
It looks like you did, but you don't say anything, nor is it written anywhere. I know this is an older video series, but I am honestly having a hard time following you, unlike in the earlier videos. Wish you could clarify what you are doing in the text, even if you can't record a new, slower video. this series has introduced me into the world of 3d printing, but this lesson has me stumped all weekend long