A kingfish of 85cm say approx 4kg can pull anywhere from 1 to 12 kg of drag pressure, many people think if a fish is 4 kg it can only pull 4 kg of pressure which is obviously incorrect. There isnt a set rule as to how much more a fish can pull above its weight, so many factors come into play, With kings there general health effects them, if they are well fed they will have more fight, If the water temp is cold it will make them more lethargic and much easier to fight, I pulled in a 90cm fish during winter that didnt pull any drag at all and the drag was set to 4kg, then i can catch a 60cm rat in summer that pulls out 7kg of drag......
Now with the water bottle test its not accurate as its a dead weight and a quality fish like a king is not a dead weight, They jerk left and right, They pull and stop...then pull again and when testing your tackle you wanna mimmick the fish as best possible, The best way to test it is to get a quality set of scales, Like the blue Shimano ones and tie a snap swivel onto the end of your line, Then lock your reel up as tight as can be and then put the scales onto the end of the line and get a mate to pull the line out using the scales, Also get him to pull in jerks so you canm test your line, Trace and knots strength.....this way you will get an exact reading of what the stella is capable off.
But before you do that i recommend you dont do it with the Braid composite rod....You said you have a 20kg braid composite rod....
The Rapala rods come in 2 of the larger sizes....a 15-24kg and a 10-20Kg........the 15-24kg is more of a 15kg jig stick and in my opinion may break if 20kg + was applied, The 10-20kg stick is more of a 10kg jigstick so once again in my opinion it will break under heavy pressure.