Variable,and set the bit rate to say 6-8100
From what I have read on this. if the picture is static and not a lot going on the bit rate is lower,and when something happens the bit rate will rise up when something is happening in the frame,also if you have your frames per second say at 25 match your I-frame rate to 25,or what ever your frame rate is match it to the I frame rate for a fluid motion in play back.