I think the most important thing is that you have air flowing through the case and warm air being exhausted out the back. You can try to get the intake and exhaust balanced, but as long as you have the air flowing I think you'll be okay. Don't obsess over it.
On my own computers I only install 1 exhaust fan on the back of the case and no front intake fan. I can feel cool air being sucked into the case from the front and warm air being expelled from the back and I have no heat related problems.
Cricket