The trick for easy going in draft is to set CC and only click +- 1 mph when needed.
This way no gas pedal fanning and my foot is always ready to hit the brakes.
I follow with about 50' distance, what gives me plenty of room even for emergency braking.
Physics say that the front vehicle takes as much of front draft as rear vehicle takes the rear draft. So my tailgating saves the semi some fuel as well, but doubt in the amount he can notice, when for me tailgating at 70 mph gives 20-25% fuel saving.
Again, I don't do that often as my main purpose is to see the country, but when you drive 1500 miles in Texas having the same landscaping, that is where technique becomes handy.