Ok a couple of things.
1 All this wasted alcohol makes me sad we should save it up and distill it, all it takes is a pressure cooker with a hole in the lid hooked up to a condenser and bam moonshine. Russian Style
2 For the purposes of making co2 any yeast will do, historically bakers got their yeast from brewers to make the bread and that is why bakers and brewers were so important in medieval europe, also why you cant swing a dead cat and not hit some one with the last name of brewer, brewbaker(or some variant)
3 Yes a 25# tank of co2 only runs you about $20 to refill, but the tank costs $40-70 a decent regulator for horticulture is at least 50-70. or you could drive around behind any place that has a soda fountain and steel the shit if they havent secured it.
4 there is a way to calculate but it is assanine
step one run a hose from your bung/airlock to a graduated measuring device e.g. Erlenmeyer flask mason jar or a 2 liter with lines marked on it
step 2 fill jar with water turn it upside down and stick it in a bucket of water, with the opening to your measuring device upside down.
step 3 measure how much it fills up the jar in 1 min or how long to fill up said jar
step 4 multiply the volume by 60 and that is how many volume metric units per hr or if it took less than a min take the volume of the jar devide by the time it took in seconds and then 3600 and that will be the volume per hr
step 5 find the volume in the grow area
OK now this is where an estimation scheme comes into play
if we figure that there is no excange of air between your grow area and the rest of the world you could simply devide your volume/hr of co2 output by the volume of grow area (keep all the units the same) then move the decimal to the right 6 places that should give you your parts per million. if my drunken math is correct.
The problem with this lies in
is all the gas produced by yeast co2???
what about the volume of the stuff like plants lights water dirt in the grow area(that should be subtracted)
do you have a fan blowing cool air in, or hot air out?
theese will all effect your calculations but i am guessing that if you multiplied your gas output by .9 that should make up for other random gasses produced by the yeast. If your grow area is in a larger room with poor circulation you could then take the volume produced in say a 12 hrs and then devide that by the volume of the room(SAME UNITS OF VOLUME) and then move the decimal 6 places that should give you a reasonable equilibrium saturation.
But if it is open and airy with an ass load of air movement...
I would have to say the solution comes for the mouth of Walter Sobcheck, from The Big Lebowski
FUCK IT DUDE LETS GO BOWLING!
lets face it some co2 has to be better than no co2