In my opinion, if you want to only give one number for the heat vent, it should be 8.5, not 9. If you round up for every heat vent, then the total is going to be off by a lot.
People who are competing to come up with the most effective design are probably looking at this as "I want to make 5-7 reactors, and get the most EU for the resources I spend". To most of us, we don't mind making a few extra iron bars for the first reactor, because we know that they'll eventually get use somewhere else. We just don't want to spend more on that first reactor than need.
Also, we may have some iron bars already in our inventory before our first reactor! I don't think that there's any way to accommodate that.
If you do want to do something far more complicated, you could output something at the end that says "Iron cost: 112-115 ingots". That would indicate to people that if they have 115 ingots, they can build this entire reactor without any more mining. If they are making multiple reactors, the average cost will be 112. Also note that although this concept is pretty easy, it could be somewhat difficult to implement. I'm not saying it's a necessary feature, haha!