Personally, I use the total arrow length with my broadhead attached. I've heard all the arguments for not counting the tip, but I'm not convinced by those on that side.

In practical experience, it doesn't really matter. The actual number is not important. What is important, is that you're able to accurately shoot your broadheads at any distance and that you provide enough weight up front to provide some forgiveness in wind, when hitting small twigs or grasses or when hitting bone. The higher the FOC, the greater the forgiveness. On the downside (in some people's view), your arrow will fly slower as you put more weight on your arrow. I don't subscribe to the notion that trajectory is affected by weight up front, vs weight somewhere else. My shooting experiments simply don't verify this at typical hunting distances.

A half of a percent won't mean much. On the other hand, five percent can be a big deal. Personally, I would never shoot a broadhead on a 10% FOC arrow, but that's just me. I want the forgiveness that I find on extreme FOC arrows (over 20%). Keep in mind, that you can't just pick an FOC and put lots of weight up front. You have to design the arrow for that weight, at your draw length and draw weight. It's a dymanic formula that needs lots of thought before building your arrows. Once an arrow is cut to length, the only way to make different weight tips work optimally, is to adjust draw weight. Most people pop on different weight broadheads without ever touching draw weight or considering what is does to the arrow spine.