You can accomplish this with a calculation which you can use to drive your reference line. It likely won't be exact due to having to use ROUND but it gets darn close.
Check out the attached and let me know if you have any questions. This is driven by approximate y ref line.
RefLine.twbx 1.2 MB
Dan, dude, thanks! that's exactly what I'm looking for. The only issues I found was when I chose the slider instead of typing the value in.