The size of the Casting stroke is generally determined by the length of the line to be cast.
Now, what does this all mean? Quite a lot actually. You constantly need to adjust the length of the casting stroke to the amount of line that is outside of the rodtip.
Remember this: Short line short stroke, long line long stroke.
I remember a drawing of this concept when I was learning the basiscs of the flycast. I will try to explain the mechanics: 60 feet of flyline weighs considerably more than 30 feet.
Asuming that you make a well executed cast, the rod will bend deeper against 60 feet than against the weight of the 30 feet. If you don't adjust the casting stroke, you will deviate from the all important Straight line Path and thus ruin your cast. Not good and no fun at all.
The key to good loop formation is to match the size of the casting stroke to the amount of bend in the rod This is known as a Variable Casting Stroke.
Essential nr. 5: