The constant can only be used as a guideline, since the coefficient works on the assumption that line velocities are constant (Chalfont's Hypothesis suggests a coefficient that accounts for line velocities, but is incalculable on standard equipment within any reasonable amount of time), and unless stated otherwise is calculated for a 25 move limit.
When Mornington Crescent has been played, Beck's reaches a value of one.
The formula for the coefficient is rather too complex and cumbersome to explain here, but is available in any modern book on the game.
[JH]