Im trying to scale a video so that it is always 512 wide where the height changes in proportion to the original video. Once scaled, I then want to apply a watermark/overlay
You can use the -filter_complex option with the scale and overlay filters:
ffmpeg -i input.mp4 -i logo.png -filter_complex "[0:v]scale=512:-1[bg];[bg][1:v]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2" output
-c:a copy if you want to stream copy (re-mux) the original audio instead of re-encoding it. This is useful if your input and output container formats are the same.overlay=10:10overlay=W-w-10:10overlay=W-w-10:H-h-10overlay=H-h-10:10