Really is impossible to say without lots more info - eg how long is the TT? How fit are you? What sort of time are you realistically aiming for? You need to be more specific about what you mean by wanting a 'better ratio' - what would you like to change about the 42x14 you used before (eg was it too heavy a gear or were you spinning out too early).
FWIW the one and only time I did a TT on fixed was a 10 miler, on a conversion with regular road bars, spoked wheels, on a flat but windy out-and-back course and I used a 44x14 (ish). I did something in the high 23 minutes (hey I never said I could TT...
) and this gear was about right for me. Based on that, your 42x14 sounds closer to the mark than the 50 etc you mention...
mickster