The 25% figure you calculated has a name. It is called the
dead-cube take point. In unlimited games (i.e., money play), whenever the game will be decided on the next roll, it gives the minimum percentage of games the taker must win in order to accept a double. So long as he can win 1 game in 4, he has a take.
Most doubles, however, occur with a
live cube. That just means that after taking, the game will continue for some time, and the taker may have an opportunity to redouble if things turn around. Because most of the taker’s losses will occur on a 2-cube, while many of his wins will occur on a 4-cube (after a redouble), his
live-cube take point is a few percentage points lower than his dead-cube take point. In a smoothly changing, non-volatile game, it can be demonstrated that the
theorectical live-cube take point is 20%. In practice, it is hard to time your redoubles well enough to reach that ideal figure. Most experts, therefore, use 21.5% as their live-cube take point in unlimited games.
But even that does not tell the whole story. Your take point must be adjusted further in order to take into account the effects of gammons and backgammons. The numbers we have been discussing thus far assume that neither side will win or lose any. When you factor them in, the result is called the
gammon-adjusted take point, but I’ll leave that calculation for another day!
If you are interested, you can search for any of the terms above, and in addition, others, such as
recube vig,
gammon price, and
doubling window. You may also enjoy reading this
old thread where some of the concepts appear.
Hope this helps.
Last edited by Taper_Mike; 04-11-2013 at 06:32 AM.