What yeast are you using?
EC-1118. The winemaker I talked to said it is perfectly comfortable at 17-18% and that 20% is quite possible with it. I'm using three packets for 3 gallons; I realize that's overkill, but you can never have too much yeast IME...
Easiest way to do it, is to ferment to dry, fortify, then backsweeten to taste. There are calculators out there to tell you how much brandy you will need to add to get to your desired ABV level.
w w w.vinoenology.com/calculators/fortification/
One thing to note - if you are backsweetening and you feel that your yeast will take off again with a sugar addition, you will want to add potassium sorbate to prevent the yeast from starting off again.
Normally the brandy addition will do the job - but you stated that your yeast strain is good up to 20% abv - I've never seen such a strain, but I suppose they do exist. In my ports that I've made, I've never had to worry about it because the yeast strain only was good up to about 15% abv. I added 1.75 liters of brandy to around 6 gallons of fermented cab juice, back-sweetened to taste and let sit on hungarian oak cubes for about 5 months. Turned out spectacular.
Thanks for the link! That was my big question--which will result in a better overall port/wine: completing fermentation alone, completing fermentation and then adding brandy, or fermenting to around 1.10-1.08 and adding the brandy then. I don't have oak to age with so it'll all be carboys, but I plan on racking 3 or 4 times to allow to age/clear completely before bottling. I wouldn't be against picking up a barrel, but they're a bit pricy for an experiment and I'd rather wait until I knew for sure that my recipe was going to work properly.
I wasn't planning on making an F-pack, but if I did I'd probably use the remainder of the Syrah concentrate and maybe some blackberries, to get a very sweet and complex flavor.
And ratios:
With the jam bag of fruit, water, must, sugar, and concentrate the volume seems to be around 3 gallons (4 gallons in the bucket, but of course the fruit is adding to that). I bought two 1.75litre bottles of brandy (just in case, if I don't use it all I have it for other experiments), but by my math (and suggestions from local homewinemakers) that should actually be the right ratio. It's being racked into a 3 gallon carboy, but I think I'll pick up a 1 gallon one as well for the extra liquid, then I can top up with it on the first rack/transfer (I plan to use the brandy to top up any future transfers).
Does that all sound sane? I've never done a port-style wine or fortification, I've only made plum wine and wine from concentrate (malbec, turned out ok).
Also, just an afterthought, but should I add any tannin? I'm thinking 1tsp if any, not sure if it will help or hurt, and not sure at which point to use it, especially if I stop fermentation...
EDIT: just for ease/clarification, here's the recipe on day 1 (last night):
Roughly 15lbs chopped plums (mashed through the open end of a LARGE jam bag)
roughly 3 cups syrah concentrate
2-2.5 gallons water
1.5 tsp pectic enzyme
3.5 campden tablets, crushed (used a mortar and pestle, so was concerned about loss from what stuck to the rock)
6.5-7 lbs sugar
Again, starting sg=1.14 (will remeasure tonight before adding yeast, but by math that should end up around 17%)
EDIT2 (done), Day 2:
measure SG: 1.13; added 1.5lb sugar, back up to 1.14
added warm water/yeast mixture (used two packets, saved one in case I need to restart)
added roughly 3/10-1/2 tsp tannin
added warm water/yeast nutrient mixture
Stirred, set airlock
I'll update as it progresses, but there's still the question of when to add the brandy (and how much to add), that's the part I'm a little lost on as the above is MOSTLY my standard for fruit wine (a couple exceptions, like skipping the acid blend, adding the grape concentrate, volume of water, SG, etc). Thanks for the suggestions thus far! Anyone with strong opinions on brandy vs everclear, volume, stopping fermentation vs. completing and adding back sweetener, etc. please chime in.