Ok, let me throw some other numbers at you. If you're a 25 year old male, you should wear size 10 shoes. Your inseam should be 32 inches. Your waist should be 29 inches. You should wear a size 7 1/4 hat.
Do ALL of these other measurements accurately describe you? If so, go ahead and use the heart rate "formula" to find your max.
If not, however, you'll see how silly the pre-set formula is. I'm 40 and I hit 193 max HR a this summer. I'm also 6'6", so the 'average measurements' for an American male don't apply to me either.
The only way to find your max is to go out and hit it. You may not even get there the first time you try to find it. Set your garmin so that it shows absolute numbers, not percentages. Whatever your max was last time (107% of whatever) use that as a starting point. To really find your max, you need to find a big hill and go up it as fast as you can. When you're about to pass out from the effort, sprint all out. If you don't pass out or puke, look at the number. That's pretty close to your max.
Your max could lie anywhere from 150 to 230. Everybody's is different.