The 20% abv is right. On the sweetness, it's hard to tell. It sounds a bit high to me, but I don't make a lot of sweet wines. I'd be inclined to start a good deal lower than that, and sweeten to taste. You'll want to do some "bench trials" -- add measured amounts of sugar to a couple ounces of wine at a time, and see what sugar level works best. If you are testing with table sugar, I believe you can expect some increase in perceived sweetness, as it breaks down into simple sugars. If that concerns you, you can sweeten using simple sugars instead, or follow the procedures available from various sources to "invert" the table sugar into simple sugars.
In terms of acids, I believe you'll want to be in the "normal" range for acids in table wines, but probably towards the high end. The sugar in port tends to balance the acidity, so you can probably go higher in acid than for other types of wines, without the final product tasting unbalanced. Normal for a red table wine would be something in the range of 7 g/L or so. If the wine tastes "flabby" to you at that level, after adding the alcohol and sweetening, you might want to do bench trials to see if a bit more acidity would improve it.
I agree with you that it's better not to be too wild and crazy with your first couple of batches, until you develop some sense for what is "normal." On the other hand, at this point in the process, the adjustment of sweetness and acidity really only have to please your taste buds - there aren't any particular "standards" that you need to worry about. It it tastes good to you, that's good enough.
Doug