Another, perhaps more accurate way to measure is this:
Take a strip of vinyl, say 36 inches long by 2 inches wide. Starting in the middle of a standing rib, continue to apply the vinyl and try to get it to end atop another standing rib. You may need more or less vinyl to accomplish this, but make it an easy number to remember.
Then, measure the overall length of the applied vinyl across the tops of the ribs. Divide the original vinyl length by this measurement. Hint: the number should be greater than 1.
Multiply this factor by total length of your intended installed design to obtain your total cut vinyl length.
This method is especially helpful if you're aligning multiple panels, since you will have accurate intervals by which to tile your layout (preferably seaming at the tops of the ribs).
JB