Shooting a framing chart only helps if it corresponds to markings in the VF. If you have access to a camera technician, you could ask them to remove the ground glass and temporarily mark it with scotch tape in a 16:9 ratio. This often done with older 16mm cameras, and works pretty well. You just do a center extraction in post. If you can't get this done, the other option is to use what is called a 'Top-line' extraction. This where you frame with normal headroom in the 4:3 frame, but make sure there is nothing important in the bottom 1/5 of the frame. Then, in post you extract the 16:9 frame using the top of the 4:3 frame, and cropping the bottom.
Actually, I took a look at your ground glass. I put the image in Photoshop, and put a 16:9 box over it, and I'd say that if you were to use the top of your existing frame, and then use the lower white line on the left side as your bottom of frame, you wouldn't be too far off. Down and dirty, but it would work.