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Let’s assume g% make game or better, d% down 1 and dd% down more then one. 

In order to gain by bidding game:-

For non-vulnerable       g > ( 5 * d + 2 * dd ) / 6

For vulnerable g > ( 6 * d + 3 * dd ) / 10 

You must simulate each situations to find g, d and dd.

For 9 HCL points against 15 HCL 1NT opener with no 8 card major fit, you should not go to game. Computer simulation result for NT contract is :-

 0 = 0.00%  1 = 0.02%  2 = 0.03%  3 = 0.09%  4 = 0.39% 5 = 1.74%  6 = 7.65%  7 = 22.20% 32.11% 8 = 36.83% 68.94% 9 = 24.85% 93.79% 10 = 5.76% 99.55% 11 = 0.43% 99.99% 12 = 0.01% 100.00%

where g = 31.1, d=36.8 and dd=32.1.

For 9HCL point hand and 4 or more cards in one or both majors you should investigate a 8+ major fit except 4333 distribution. If you find an 8+ card fit in the majors you should go to game.