How long the tank will last depends on the CFH (cubic feet per hour), the calculator doesn't take into consideration the dimensions of the room because thats not the only variable, you have to know how much co2 your plants are using and how much is leaking out of the grow space and shit like that. You wanna figure out what the least amount of time is that you can be pumping co2 into the space and maintaining 1500 ppm or whatever you're aiming for.