Don't overthink it. You can work
it out after the fact. If you plan on having 2.5 against you, so that pretty well determines when you'll leave, and then you sail your boat and have fun, then check your arrival time. If it's earlier than you planned on, then it was less than the 2.5, and you had a good trip. The weather
and river gods give you whatever they feel like at the moment, and you get to play with it.
It is that where it narrows, it will run more strongly against you, wider areas, not as much. Your tacking angles, as shown on your GPS
, will not be nearly as tight as in water
with little contrary current.
of course, if you like doing the arithmetic, and are willing to pour over the chart, and compare the depths and distances, you could probably make a drawing showing the current against you at the narrower places and see if that's what you get. For me, sailing is more about the interaction with the elements than the intellectual aspects, but everybody's different.