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

JON-A-TRON23 days ago

I think you're talking about the tolerance between the hinge parts. In general, you have to think about sketches in terms of closed profiles- that's the only way they're useful in making 3D geometry. If you offset an arc, and it's not long enough to make a closed profile of .5mm, for example, you'll need to extend it to make it work.

You might find this video of a webinar I did on it useful: https://www.youtube.com/watch?v=w4XnonJ55Ag&t=850s