As there is no secondary fermentation when brewing in kegs as you use CO2 gas, how do you calculate the added amount of sugar in the brewing process to achieve the same end result. Sue
When serving beer from a keg, there is no need to add sugar after fermentation, as you mentioned, no secondary fermentation occurs so it is not needed.
Instead, you set the pressure of the CO2 in order to obtain the carbonation required at the serving temperatures. I personally use BeerSmith to calculate this value for me.
If you are referring to keg or cask conditioning, the rule of thumb from Papazian's Joy of Homebrewing is
1/3 cup corn sugar per 5 gallons. ... Excessive foaming will result from the normal rate... The keg is then sealed and set aside for 1-2 weeks.
(At room or cellar temps, NOT in the fridge.) Good luck.