The process is not exact, as it will vary according to the health and strain of the original yeast, alcohol level, length of aging, etc. But there is good information in Yeast (White & Zainasheff) pages 115-118 about bottle conditioning. They discuss yeast re-pitching rates of between 1M-5M cells/ml in filtered beer, depending on the age and alcohol level of the beer.
Since most homebrewers do not filter, you have to try to account for the remaining yeast population. Mostly, you'll be guessing here. I have had good results with higher-alcohol Belgian beers which I had aged for 2-6 months, by re-pitching at a rate of 1M cells/ml. On some of them, I wished that I had gone a little bit higher. When I first did this a couple of years ago, I did some math based on the volume of a White Labs vial and the average cell count as stated by White Labs. It worked out to about .27 tsp. of White Labs yeast per gallon of beer. (Caveat: I entered college as a pre-med major and exited college with a BA in Political Science, so you might want to do your own math.)
One other thing you can do, is to rack your beer out of the barrel into carboys when it has the character you want. Then bottle just a portion (maybe a gallon) of it using whatever pitching rate you choose. Let it carbonate at 65-70° for awhile (probably at least 3-4 weeks, big beers can take a long time to carbonate), and based on the results you can adjust the yeast pitching rate for the rest of the batch.
Last thing I suggest is weighing your priming sugar rather than using volume measurements. I use about 145g of sugar per 5 gallons of finished beer for the Belgian styles I have bottled this way in the past year.