Use 19.7Mbps just like your source clip. That's actually fairly low for a 60p HD clip, as PD's AVC MP4 default export profile is 40Mbps. The Best Matched Format should create a custom profile for you and the quality should be the same as your source clip.
Using the default profile that I described, or increasing the bitrate beyond 20Mbps for any reason will not improve your quality and will only give you bigger file sizes.
As tomasc mentioned, you may even be able to lower the bitrate to reduce the file size, but I don't think you should go below 15Mbps. Personally I would match the bitrate and not think about it anymore.
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