i do use a calculator, i believe the brewersfriend.com one, but just my estimate of 1/2 to 2/3 since YMMV and i believe it is not so easy to gauge what a brewers CO2 vols are while bottle priming. it all takes dialing in to ones system and to what one likes. ie. its not easy to just prime a beer using the calculator according to what you believe the residual co2 in the beer is and know it is exactly 2.4 vols. so what i believe to be 2.4 may in fact be anywhere from 2.1 to 2.7. i just dial it in to what i enjoy and believe are my numbers.
I let the beer ferment for at least 18 days in a small bathroom (having a south-facing exterior wall) that is cooled by my central A/C. My limit is 20 days. During those 18 to 20 days, I’ll have my A/C set to 72 degrees the entire time. There is a stick-on thermometer on the fermentation vessel, so I will be able to monitor the temperature while I am home. I do overnight trips that usually have me away from home one night per week, and two daylight runs that have me leaving home around noon and getting back around midnight. It hasn’t begun to get really hot in the Reno area, but it’s coming…
If using a priming calculator, be sure to select the type of sugar that you are actually using (table sugar, corn sugar, etc.). Same if you are using a rule of thumb like dmtaylor uses. There is a big difference in amount needed with the various sources of sugar.
The Brewers Friend calculator acknowledges this uncertainty:
“However, if the beer was cold crashed, or put through a diacetyl rest, or the temperature changed for some other reason… you will need to use your judgment to decide which temperature is most representative. During cold crashing, some of the CO2 in the head space will go back into the beer. If you cold crashed for a very long time this may represent a significant increase in dissolved CO2. There is a lot of online debate about this and the internet is thin on concrete answers backed by research. We are open to improving the calculator so please let us know of any sources that clarify this point.”
I would love to have something more definite, but it is what it is.
There is so much information gained from actual experience in these pages, I feel very fortunate to have been allowed to join the site.