There is a minimum and maximum temperature,have a look in your fcom.(perf).
If you cant insert an ass temp less than 32c,it means that for the rating,pressure altitude and outside temp ,your thrust is at maximum.
If you assume is too low,it would give you a thrust higher than your pressure limit and the fmc will not accept,as you increase the assume input,you will see it will accept when the thrust value is lower than your max thrust for rating.
Have a look at your climb limit weights,they will decrease by about 1.2T until a certain temp then remain kind of constant,it happens at your min temp for rating and temp and pressure alt.
I hope it helps.
Last edited by de facto; 4th Jan 2013 at 21:21.