The concept of fortifying and calculating the alcohol level seems easy enough, but as they say, it's easier said than done. Considering the small volume you are working with (3 gallons), your measurements need to be quite precise to avoid over-doing it. Fortifying affects not only the alcohol level, but dilutes/modifies the mouthfeel, 'wine' flavor and overall quality, in ways that are impossible to describe in words (fortifying with Everclear versus brandy/cognac results in very different 'finished' products, in numerous ways). On the other hand, not enough fortifying and you won't notice any difference, and will be frustrated that you wasted your time and effort. Another variable is that it takes an unpredictable amount of time for the fortifying addition to integrate with the port - maybe 6 months, maybe 2 years.
Having not done it with the Porto Corinto before, I didn't want to potentially ruin the whole batch, so I decided to fortify only a portion - which left me with the problem of separating that portion and knowing how many bottles that would fill (you know, talk is cheap; doing it can be a whole 'nother ballgame). After looking at my carboy ready to fill 20-odd bottles with port, the measured out cognac and my calculations of how much to add to ~40% of the 3 gallons but not knowing how to divide the port into another vessel without making a mess and spending 2 more hours on it, I gave up. In the end, I simply added a shot (~1.5 oz) of cognac to 9 or 10 375-mL bottles, and crossed my fingers. I like the finished product, but I wouldn't recommend this procedure to anyone else.
