If your vehicle warranty says "only up to x% BD" they still can't void your warranty just because they find a higher percentage of BD in your tank. They must demonstrate that the fuel caused the problem. That's not to say they won't fight tooth and nail to avoid it, but it is true. If you are denied warranty coverage because of BD, demand to see the results of the fuel test that demonstrated BD caused the problem and call a lawyer if they resist. Once when I had my chipper in (because it had a clogged fuel filter, it turns out, but the machine was pretty new and I figured I may as well use the warranty) they blamed the fuel. I knew better and told them so. They persisted. Eventually I said, OK, test the fuel. Suddenly they were ready to try a new FF first (which I told them was the likely problem when I dropped it off). Then they charged me about 3-4 hours labor for the job. I pointed out to mgr that the FF is on the outside of the machine and can be changed in about 5 minutes. He talked to mechanic and reduced the labor by 2 hours because it turns out they were charging me for his time on the internet researching BD. I still paid too much for the FF, but it seemed a reasonable compromise.
Worth noting that the same is true of #2 petrol--if you have bad fuel and it causes a problem, the dealer will not cover the repair.
I use commercially produced fuel (when I can get it--local supplier is going through problems now). I may homebrew eventually, but now I need a better place to work on it before setting up all the equipment.