I just upgraded to a 4x4 what are you using for a humidifier mine is struggling to keep it at 50%. Also might be a dumb question but do you all open to flaps on the sides or just one?
A normal sized mist humidifier (.5-1 gallon) should do the job, but depending on your humidity and temperature levels you should expect to go through a gallon or more of water a day during veg to keep it at 50% especially in a dry climate.
As plants get bigger and you add more plants they will help keep the level up naturally, so if you're working with small plants/clones right now that is part of the low humidity. Flowering will also need less water and humidity.
Also consider turning down your exhaust. Many fans can circulate the air through a tent multiple times a minute at full strength, so turning it down to the point that you see a slow drop in humidity when the humidifier turns off can help not suck away that water.
Feel free to open all flaps if you're not worried about the light coming in/out. Try to even the intake flow on each side of the tent to prevent low pressure areas from developing when certain corners don't get the air they need (also often made worse by too much exhaust).
I usually try to keep it at a ratio of 2:1 intake to exhaust. Meaning if i have a 4" fan i like to have 2 intakes that size as well.
12.5 sqft? What is the height of your area? That's like a 3'x4'x5' tent I'm amazed you'd need that much. I usually need a gallon for that size, coming up from 20% rh.
Dang man you're using a ton of water for that size of area must be super dry where you live i'd guess you're filling it every other day or so though right?