Total airflow THROUGH the case is the target. (Ensuring the flow gets to most of the case volume is secondary but important.) Then recognize that what goes in MUST come out. Hence whatever you do to force air IN should be balanced by some similar effort to force air OUT. If you not do that, the actual balance of airflow WILL happen, but it will be impeded by lack of assistance in your design.
As a STARTING point, the number of fans in each of the Intake and Exhaust positions PLUS the characteristics of each fan (easy if all fans are identical) should match. To be a bit more precise, he max Air Flow specs of the fans should balance. HOWEVER, the actual air flow each fan can deliver also depends a great deal on what items in the airflow path impede that flow. Various components inside the case affect this. So do foam dust filter sheets in front of intake fans (and especially if these are clogged and not cleaned). Finned heat exchangers (as on a fan-cooled CPU cooler) and in a radiator offer more airflow resistance than any other component. For these latter devices one can choose to use fans designed to force air flow against higher flow resistance (called Pressure Fans). The exact impact on actual airflow from these factors can NOT be predicted, but you can "bias" the flow estimation calculations by trying for slightly more intake capacity than exhaust.
Once you have your system built and operating with default settings for fan controls you then can do some actual testing of real airflow balance. This gets us to the concept of case air pressure, positive or negative. If the forces pushing air into the case exceed the forces pushing it out, the air pressure inside the case will be VERY SLIGHTLY higher than in the room outside the case, with the result that air will leak OUT of the case at any small cracks. This can reduce substantially the entry of room air at those cracks. That is important for reducing intake of room dust in that leaking air. Thus as long as all your INTAKE points have dust filters in place (that are maintained and cleaned periodically) you can protect your system from excessive dust accumulation. THIS is the main reason for aiming for "positive pressure" inside your case. That condition does NOT affect cooling directly - the impact is indirect only since it reduces the insulating effect of dust on internal components.
You can test the air balance using the smoke tracer technique. For this you need a small source of smoke such as an incense stick or a smouldering cigarette, and a small variety of jobs for the computer to run for a few workload levels. For each workload get the machine running and move the smoke source around the outside of the case, observing the smoke plume flow. If it is sucked into the case the interior pressure is negative and dirty air can flow in. If it drifts away from the case the interior pressure is positive. If the smoke blows very fast in either direction, the pressure difference between interior and room is too high. This should be repeated for two or three workloads to get a complete picture. Large imbalances may require adding or removing a fan. Modest imbalances can be corrected by altering the "fan curve" settings for particular fan groups from default to some custom settings. For that purpose it is useful to arrange intake and exhaust fans in different groups on separate mobo case fan headers. IF your system uses a liquid-loop cooing system with a radiator (AIO or custom) that also will make a substantial contribution to overall case ventilation. The settings for that system must be optimized for its cooling function on the CPU chip. So adjustment of overall case airflow balance must be made on the case fans.
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