Yes the controller is the best way to not waste CO2 which translate to saving money for you....... But you can just mutipli Legthxwidthxheight to get you cubic volume then find out how many cubic feet of gas your regulator is going to release in a set amount of time and you can guesstimate the entervulls to put your timer on to release the Co2....Other then that your only other choice is to get a Co2 censor and that must be connected to a controller and the controller is connected to your Silenode/regulater....
Check out my homemade Computer Prosseccor Control panel...... there is no timers on my grow it runs off a prossecor like your desk top... Everything is controlled from this one panal....Co2,heater, AC, fans, lights, dehumidfier, its all controlled from inside the house from laptop.....I set the setting from inside on the computer that is sent to panel in the grow room and then it does everything for me based of how the software was writtin......
My dads an enginer so we made it with spare parts from his work and a demo software he jacked......