co2 question

well it is that time of the year again, I am doing a new set up. This time we are using the old school tank set up with a art DNe Recycle timer. The room is a 15 x 20 and I have been told to run every 30 min for 15 min. seems like alot of co2. I have tried the co2 calculator but all it says is setting 1.

room size again is 15 x 20 with 9 foot ceilings. thanks for any help in advance

well I think I got it tuned it.. hard work using a tank set up properly.. I have the timer set to run every 15 min for 3 min at a time.. I know its just numbers with out a ppm monitor. any pointers on where I can score a good ppm gauge