Sorry 'bout the delay in response Bill. Still trying to wrap up things from crush around here (and try out a new news reader!)
Anyway, I've tried some of Les' port style wine and it was wonderful. At the time, I thought it was only Foch, but tremendous none the less for a ruby port style.
Concerning when to stop a port, ruby's tend to have between 8 -12 % sugar, but the exact desirable endpoint is, as you would describe, dictated by taste and therefore dependent on the TA. The fortification should be done several degrees before the desired target. In other words, if you want 9%, hit the must at 11%, as it will take awhile for the yeast to actually stop.
I've pondered, without any real answers, about the drift time between when the alc is added and fermentation stops. Theoretically, if the calculation for desired alcohol is correct, the fortification can take place anytime before the endpoint, and the fermentation will stop at the proper point, by definintion, when the alc/sugar/pH levels become toxic to the yeast.
I've yet to do a production port style yet, but have done my second experimental batch this year (doubled the volume to 10 gallons) using Norton, which I affectionately call it "Porton".
clyde