For anyone who's interested I worked it out.

I created a second series on the second Y-axis.  This
series is equal to the highest point on first axis, less
the value of my dashed line.  I made this series invisible
(no line, no shading).  

Since it's an area chart my 'invisible series' sits on top
of my dashed line, and together they always sum to the
highest point on the first axis - therefore Excel sets the
second axis to the same scale as the first.