If my calculations are correct, you have a cayenne percentage of 1.67%, which won't add much heat at all. The chili powder is @ 9.68%, which is fairly well balanced for a spicy, but mild heat.
The red pepper flakes don't add much heat to the food until you chew them up, so keep that in mind.
The black pepper adds a bit of spicy heat, and that's a ratio of 4.92%...my milder rubs have about 9-10%.
The brown sugar will be fine for pork ribs, beef ribs, chicken, chops, steaks and other shorter smokes (watchout for high temp grilling, though), but I'd pull it from the mix if smoking a butt, brisket or anything else which will run over 6 hours total time to smoke/cook, as sugars will carmelize after just a few hours, and shortly after will burn even at lower smoke chamber temps of 225* or so.
I used to put light brown sugar in my spare rib rub and it worked out fine. Dark brown sugar is OK too, it just turned very dark after a few hours into the smoke, and I wasn't sure how long it would take until it started to burn, so I only used it once. If I recall correctly, it was about 2/3 the ratio as your recipe calls for. I haven't used sugars in my rubs for almost 2 years now, due to a diabetic the family.
Some folks don't like cayenne for heat because it can leave you with a slightly bitter after-taste. I offset this with about 8-10% cinnamon by volume of cayenne pepper. If I want to pack some heat with a great flavor, I grind up a few dried ancho chilis for some heat and a great flavor not at all like the red chili in bottled chili powder, then top off the heat level with cayenne pepper and a pinch of cinnamon, and we're off to the rodeo! Ha-ha-ha!!!
Oh, if you want to try the ancho chili route instead of red chili powder and pack alot more heat, I would double your amount and omit the red chili powder, add the bit of cinnamon, and double your cayenne. You should be able to find whole dried ancho chilis in the ethnic section of most grocery stores, near the mexican foods (taco shells, salsa, etc). Oh, I freshly grind as much of my ground spices as possible with an electric coffee grinder, but don't ever use the grinder for coffee again.
You won't like the wild flavor of your coffee for days afterwards if you do......and your significant other half will want to. Two grinders: one for spices, one for coffee
Great smokes are coming your way...playing with dry rubs is one of my favorite past-times, 'cause once I get a new one formulated, I gotta put it on some meat, smoke it and eat it to find out if I got the new flavor craze I was looking for.