Dave, welcome to The Planted Tank, thanks for asking this question.
You are right, there is always a little bit of solution left over that can not be pumped into the tank.
This amount can be minimized by using a pump that sucks the water from its bottom (like the Hydor Pico) or lay the pump on its side so the impeller compartment is on the bottom (make sure the water can still circulate though).
You could also use the leftover bit of solution to "gap dose" after a water change, if you do the water change and fertilizer refill on the same day.
I usually end up fertilizing my house plants with whatever is left in the containers.
For my 10gal tank, I use a different method with an airpump pushing the solution into the tank, and there it will be used to the last drop.
Regarding adjusting the strength... I don't think this needs to be THAT precise. After two weeks of dosing, only a small amount remains in the containers, I guess you could slightly increase your fertilizer portion, I figure it doesn't matter.
Let me know if there is anything that should be explained in more detail. Like Rex says, dosing fertilizers can be a challenge, sure.