What I do usually, is drop the default CRF down about 3 points below the default. I don't worry to much about the starting size of a file other than a rough guess of quality if I don't know how it was created. The Bluray media can use a worse constant quality setting because there is a lot more bits to start with and with more detail the algorithms have more flexibility. Do you'll see a default of 23 for DVD quality and 28 for Bluray quality which might seem backward but it's not. In CRF a lower number means a "better" result. Switching to Constant Quality makes sense because it guarantees that you get consistent results from file to file without having to know the media well and works great in batch processing of files. So in scenes without much detail or change the bitrate needed to support the compression will be low but is fast changing scenes as often found is action movies more bits will be needed to hold the constant quality. It will use whatever bits is needed to hold detail at a specific quality level. With the constant quality setting you never know how small your file will convert because the conversion is based on quality not bitrate.
For Bluray or higher quality generally a 28 is recommended by I use 23 to 25. For DVD quality a CRF of 23 is recommended but I preferred 20. Instead of converting using some bitrate threshold you instead do you conversion based on a quality level. The idea of this mode and what I always use when converting to save storage space is this Constant Quality mode. In xmedia Recode this will be the "rate control mode" setting and "Quality" setting. That was supposed to say CRF (corrected) which is short for Constant Rate Factor.
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