Reference Line
Step 1: Length
Go to the Analyze tab and use the Length command to measure the length of the reference curve.
Step 2: Curve Length
In this case it is 75.067 millimeters.
Step 3: Line From Mid Point
In the Curve tab, under the Line submenu, select Line From Mid Point.
Step 4: Create the Line
From the Top View and with Grid Snap activated, select a center point below the elements.
Now, type the length of the curve followed by / and 2 to divide the measurement and create the correct line length.
Step 5: Analyze
Analyze this line to ensure it has the same length as the reference curve.

Video Lesson
Next Step: Insert the reference image and adjust the curves to start building the pattern.