One could argue that anyone who has difficulty in judging in how much they can drink before they fly (both in quantity and time) has got an impaired judgement and should have a hard think about their ability to fly.
After all most flying accidents are not the result of people not being able to do the physical thing but from a mistake in judgement.
It is not just the alcohol which has to be taken into account. When alcohol gets broken down by the body a number of byproducts get produced which not only can contribute to the impairment of the airman/women but also give other side-effects. Furhthermore alcohol works as a diuretic (removes water from your circulation) which contributes to the dry feeling after a night no the razzle. Dehydration in itself gives symptoms (headaches etc) and causes lack in performance.
Alcohol concentration depends on intake and elimination and surprisingly may be gender. Blokes are better in this respect as pound for pound they have a lesser bodyfat percentage than wimmin. Elimination rates of alcohol depend on a great number of factors constitution and ability of your liver to cope with the stuff (which is partially genetically determined) Further liver enzymes can be induced by previous alcohol exposure and the bloodflow through the liver increases by 50% by lying down.
Drinking coffee does not help in reducing the chance of having a hangover as it does not rehydrate you. In fact the caffeine is a further diuretic which will only make matters worse.
It is boring but the best way to cure a hangover is by not drinking too much in the first place.
Flying ability does not just get impaired by alcohol. Other drugs (recreational and prescribed) are equally responsible for a reduction in ability. The fact that the vociferous argue the contrary is proof that they do affect one thing in particular, self judgement!
Have fun
FD