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The cube efficiency is obviously an important parameter, unfortunately there haven't been much investigation carried out, so GNU Backgammon basically uses the values 0.6-0.7 originally suggested by Rick Janowski:
| Position Class | x (Cube efficiency)
|
| Two-sided (exact) bearoff | n/a
|
| One-sided bearoff | 0.6
|
| Crashed | 0.68
|
| Contact | 0.68
|
| Race | linear interpolation between 0.6 and 0.7
|
For race GNU Backgammon uses linear interpolation based on pip count for the player on roll. A pip count of 40 gives x=0.6 and 120 gives x=0.7. If the pip count is below 40 or above 120 values of x=0.6 and x=0.7 are used, respectively.
For the two sided bearoff positions the cubeful money equity is already available from the database, so for money game there is no need to calculate cubeful equities via Janowski's formula. However, the cubeful equities for money game cannot be used for match play. Instead of using a fixed value of x, say, 0.6, GNU Backgammon will calculate an effective value based on the cubeful money equity. The cubeful MWC is calculated as usual, but with the calculated x.
There is obviously room for improvements. For example, holding games should intuitively have a lower cube efficiency, since it's very difficult to double effectively: either it's not good enough or you've lost the market by a mile after rolling a high double or hitting a single shot. Similarly, backgames will often have a low cube efficiency, whereas blitzes have may have a higher cube efficiency.