Balancing a Zoom Lens on Gimbal

I recently bought an RS5 and I’m using my ZVE-10 w/ a Sigma 24-70 on it. I tried balancing the gimbal with the lens half way at 45mm, but when I tilt it upward to balance it starts to move and go to 24mm.

I don’t have three hands so I need a way to hold it at 45mm so I can balance the lens for all focal lengths.