Bitrates: My CQMatic experience
 

These are the program versions I am using:

TMPGEnc Plus Version 2.521.58.169
CQMatic Version 1.2.06
CalcuMatic Version 1.06

Mostly, I encode AVI to MPEG-2. I know there is an accuracy issue with AVI and CQMatic...typically, the resulting video file is too large as was my experience. I looked thru other posts and common is the .57 minimum VBR option. I don't know if it works, but I don't want such a high min VBR.
I saw a suggestion to set the Calcumatic disc size option from 1 80 min cd to a custom setting of 702Mb. This made my video files too small.
So, I decided to set the custom size to 795Mb. This, in theory, should give me a CQ equal or very close to the 1 80 min cd option. Well, it doesn't. When I selected 1 80 min cd, the X3 prediction was a CQ of 74.92. When I selected a custom size of 795Mb, the X3 prediction was a CQ of 71.32. The later gave me a final cd size of 784Mb. The former exceeded my cd capacity by 80Mb.
I've tried this method 3 other times. Each time there was difference in CQ with the two methods and each time the custom size of 795Mb method was within 15Mb of a single 80 minute cd. More important, the result never exceeded the capacity of a cd. I'm content with these results.
I'm not suggesting this is a magical(and simple) solution. I realize that there are several varriables and what works for me may not work for you. I only wish to relay my experience. Try it if you like.

Good luck to all who pull their hair out because of CQ predicting.