Male = 214 - (0.8 x age)
Female = 209 - (0.9 x age)
Once you find your age-predicted maximum heart rate by your gender, then you simply plug that number into the rest of the Karvonen formula to find your heart rate training zones.
But alas, even with the gender modifications to the formula, it's still not 100% accurate for everyone. To really find your accurate Max Heart Rate, you'd need to go to a sports science lab under a controlled setting for safety reasons. That can be expensive, so most opt for one of the formulas above and rely on a heart rate monitor to keep track of heart rate.
I personally am more of a "run by feel" kind of guy. That's what I like about Runner's World's break down of the training zones above. It also includes the perceived effort scale of 1-10 as well as the talk test. I tell my runners all the time that if they can carry on a multi-word sentence conversation during a tempo run, then they're not running hard enough. Also, I already wear two motivation wrist bands, a sports watch, a runner's necklace my kids gave me, and my Garmin GPS. The thought of something else on my wrist and a strap on my chest is just too much to bare. But to each his/her own.
Heart rate monitors are great tools for checking in with your training progress. If you have medical conditions that warrant keeping track of your heart rate, heart rate monitors can be invaluable. If you're healthy, however, just try not to depend on them 100% of the time. Learn to read your body tech-free too.