I personally never heard of optifuel. Honestly I prefer VP fuel but Byrons fuel is the most popular. Odonnells and Nitrotane arent bad either but I would recommend either of the first two I mentioned. As far as percentage goes, the percentage is the amount of nitro and oil in the fuel. If your in the hobby for racing (inculdes bashing too) run either 25% or 30% nitro. Oil mixture varies by company so if you ever switch fuel brands, stick as close to the percentage of nitro and oil mixture you had previously run as possible for longer engine life (unless you have a new engine it doesn't matter what you choose as long as you stick with it if possible.) If your just into the hobby for bashing get no less than 20% nitro. Whatever fuel you use to break in the engine with is what your engine typically runs best with. If you start at 20% and want to switch to a higher percentage of nitro you may do so (note engines run hotter with higher mixture of nitro which means you would have to retune and watch your temps very closely so you get the right mixture and your engine to run at the correct temp.)
Also a little hint. A lot of people who get into the hobby think that its best to use the least amount fuel line possible. That is not true. The longer your fuel line the better. With short fuel line your engine will 99% of the time shut off and youll have to restart it. The longer the line you have the longer it will stay running upside down (very crucial for racing.) That doesn't mean you should go all out with the line either. Also before you run it for the first time I highly recommend getting a fuel filter if you dont already have one. Also make sure to clean your air filter all the time. I race and clean my air filter in between each race.