Are you using AutoCAD or a cad program to do this?
If so, you can make the computer do it for you.
Let's say you have a 150' left turn lane, so you 4" double yellow stripe is 150' long. From that point you want start your reverse curves. I usually use 300' reverse curves for urban left turns which should put you in the neighborhood of 30 miles per hour for the deceleraton transition. It will also give you around 120' of taper or gap length before the left turn lane starts.
So from the point where you want to start your reverse curve (at the 150' queue length) draw a circle with a 300' radius.
Then draw a line 90 degrees to your double yellow stripe and starting at the end point of the double yellow stripe and extend it over to the edge of the circle.
Move the circle from where the line hits it over to the other end of the line where it hits the double yellow stripe. You should now have a circle with a 300' radius that is tangent to your double yellow stripe.
Now draw another circle tangent the first circle and tangent to the double yellow stripe where you want to transition back to the normal lane width.
Trim your circles and you should have two 300' radius reverse curves with the same length.
Offset the two arcs to both sides to create your double yellow stripe.
I hope that makes sense. If not, I'll check back and try to provide some more clarity if you have questions.
sorry about that.