The HPIs use a 25 Spine servo horn. While I do not own a Flux I did just purchase a 4.6 Trophy and it came with some additional Servo Horns in the parts kit.
I replaced the the stock servo with a HiTec HS-645MG which is 133 OZ metal geared servo and the Trophy came with a 24 Spline Servo Horn along with a 23 Spline Servo horn.
Futaba uses a 25 Spline and the S3305 is a 125 OZ Metal Geared Servo. I just happen to have one laying around and just fittedthe HPIStock 25 Spline Servo horn ona S3305 Servoso I would not give you poor information.
Servo output shafts
23 tooth spline - Sanwa / Airtronics / KO / JR
24 tooth spline - Hitec
25 tooth spline - Futaba / TT / ACE / HPI/Traxxas
Hope this information helps you to choose a better steering servo