Do a set using a weight you can lift for 8-15 reps.
Then perform a set of lying (supine) front raises using a weight you can do for 8-15 reps.
The point of maximal mechanical tension when doing front raises is when your arm is parallel to the floor. So when doing the seated raises, the point of max tension is when your arms are perpendicular to your torso. With the lying front raises, the point of max tension is when your arms are parallel to your torso.
Ever had one of those days where you just couldn't get into the flow of your workout? Or worse yet, you don't even make it to the gym? It's happened to all of us before. The mind is willing, but the body just isn't ready (or vice versa).