Most times, if it isn't practical or sensible, I will ask, in plain language - why ?
Shannon wanted 270 - 20 before LIFFY this morning - the BSA is 270 AT LIFFY, I didn't have the spare fuel due to a load of pie eaters down the back on a long flight and we negotiated a compromise
I try and programme the worst of the restrictions into the box and then negotiate for better if possible.