Curve between parallel lines for Civil cell

I am trying to create a left turn lane, for example with a 6' wide median and a simple 3' radius curve connecting both ends of the median.

All this as a civil cell but I am having issues with the curve itself. I can add an "Arc between points" and specify the radius but I would like the civil cell to be editable and change the width of the median. Therefore I wanted to use "Arc between elements" which works fine, I can select both elements, 0 offsets but then it requires a "through point" which I don't want to have in my civil cell, I simply want the arc to be at the end of both my lines.

What is the best way to simply get a curve at the end of two parallel lines, but still editable?

Thanks