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The most under appreciated part of a hog barn ventilation system are the inlets. They could be ceiling inlets, wall inlets. For conventional barns, soffits serve as inlets for the attic. Setting inlets correctly will reduce your heater use in winter and power use in summer.
First, understand what your target minimum CFM are per head and set your minimum ventilation fans accordingly. Then, understand what you want for feet per minute at each ventilation stage. If you have weighted or static pressure controlled inlets this is easy because
minimum ventilation is all you need to account for; additional stages will take care of themselves. 4 way inlets do get a bit tricky, but when barns are remaining in minimum ventilation for a long time it's likely at least 2 (maybe 3) of the 4 sides can remain shut without issue.
Power actuated inlets take a bit more trial and error but shouldn't have to be checked often once they are set. Don't have a wind speed meter? No problem. The goal is for cooler air to run across the ceiling for 10-15 feet, pulling the humid, warm air down.
Using your hand or head to feel the air throw suffices as a good measurement. It may be difficult to achieve this much throw for many reasons, and it doesn't mean you have to increase fan power to get it. Many barns, especially power ventilated barns, have too much inlet space
for minimum ventilation needs. Some may need to be locked closed during cooler weather. Consider equal distribution of air to minimize dead spaces as you decide what inlets to close. I accept 300-500 FPM (5-10 ft of throw) in minimum ventilation, which can still be tough to get.
If you have trouble achieving adequate air speed in higher ventilation stages the problem may be with inadequate air access to the barn attic. Not enough soffit space or blocked soffits are common in new and old barns. Ridge vents are only for hot air escape from the attic!
On a windy day, a barn with inadequate soffit or attic opening may have a pressure reversal where air leaves a barn through the ceiling inlets and goes out the ridge vent. This is not good for trusses!
Leaks in curtains, fan louvers, pit covers or fan transitions, etc create additional "inlets". This works in the same sense as having too many ceiling inlets, limiting air throw. And having no inlets creates a similar scenario. Poor air distribution, high humidity and ammonia.
The common mistake is to turn up fan power, but why? Because it's easy! The pigs don't require additional air turnover. They aren't producing excess heat and humidity. In most cases they need better air distribution! Learning to appreciate and account for air inlets saves money!
