JABBARA - there is nothing to stop you entering whatever figure (greater than CMR) you wish into the FMC, as long as all the crew know
a) why you have done it
b) what it means
c) what you are going to do when you get the warning 'using reserve fuel'
Doing it the way your 'smart guy' says has a little merit in as much as it will alert you to the using of some of the contingency fuel, but since you are extremely likely to use some of that I would question the value - and, of course, boxes a), b) and c) above are 'ticked'? I personally do not see the point, and competent crew should NOT need a computer to alert them to unplanned burns.