Its funny, my last fillup I did put in a bottle of injector cleaner.. LOL But I don't see a need to count that among the fuel really. I see where you are going with it, but I think that is being a bit overkill. Most people don't bother with additives since most of them are not truly beneficial anyway. The only reason I put in some injector cleaner is because it needed it, not because I wanted to. I can count the number of times I have used it on my vehicles since I have been driving on less than 2 hands. The 03 van I bought was poorly maintained by the previous owner and needs more work yet, but getting better..
Anyways, not to get off topic. Do you factor in the COST of the additive and add that to your fuel purchase at the time of fillup, including the tax? That would also change your price factor. So in your case, you would put in the additive, then put in the fuel. Later sit down and do some math. You would have to figure your total volume, when you have done, and then your total fuel cost. Since you also have to add in the additive, add the cost of that, plus the tax for it, plus what your fillup was, then divide by the total number of gallons including the additive volume and it will give you a correct per gallon price, leading to a correct cost for that tank.
Though just a thought. Additives are for things like spark knock, or injector cleaning. Do you have spark knock? If so, go up one grade of gas (87 to 89 for example) instead, it will be cheaper. If you are using it for injector cleaning, realistically you should only need that at most every 15-25 thousand miles, or at least once a year. If your injectors are getting THAT dirty to need it each fuel up or something, its time to get gas elsewhere or have your fuel system checked.
Just trying to help.