Adding this scaling to existing ffmpeg code?

[Chrisj]

When I asked about adding a watermark to a video, this was suggested: "when scaling it, I'd do h/5:w/5 (of the source video), then position would be w/20 (5% from the left) and h-h/3 (75% of the way down)

When I asked about adding a watermark to a video, this was suggested: "when scaling it, I'd do h/5:w/5 (of the source video), then position would be w/20 (5% from the left) and h-h/3 (75% of the way down)"

I'd like to try that, but I don't know how to integrate that into my existing working code:

 $ffmpegCommand =''.$ffmpeg_b.' -y -i '.$video_file_full_path.' -i '.$watermark_image_full_path.' -filter_complex "scale=426:-2, overlay=10:10, " -vcodec libx264 -preset '.$pt->config->convert_speed.' -crf 26 '.$video_output_full_path_240.' 2>&1';

I've looked at the ffmpeg documentation, but need additional assistance. any help is appreciated