Hmmm, bit of a tricky one. FCO 1703 may be a good starting point as it only permits you to take more than Sword, not less. There's certainly potential to take less safely at times, as sword does work on most used runway and longest likely arrival, so favourable winds/higher levels may reduce burn a bit. That said, its not really on to completely ignore the wishes of the other crew member. Perhaps in future you might express your concern about the first incident, remind him of his responsibilites under FCO 1703, state for the record that you object to him reducing fuel below Sword without good reason and point out that should you find yourself 3mins from reserve again you'll be making a beeline for your fleet managers office to explain your concerns.