You have little time and want a quick rule? Here it is:
-Scoliosis with less than 20° Cobb angle: The general rule is wait and see. No treatment is generally needed, but close observation is. Particularly if the girl or boy has not developed yet
-Between 20° and 40° in a growing patient: A brace is the general rule. Research has shown that braces are more effective when they are worn for more hours a day. The best brace dose is 20 to 22 hours per day.
-More than 50°: Surgery is generally indicated. Scoliosis greater than 50° increase even after skeletal maturity, so surgery is preferred in order to avoid the problems associated to progressive scoliosis.
You're right, whe have left scoliosis 40° to 50°. These cases deserve individualized evaluation by an specialist. Depending on the age of the patient, the degree of skeletal maturity the aspect of the back the rotation of the vertebrae (and the magnitude of the rib hump and the position of the curve, some patients will not need treatment (when all the above parameters are on the "good" side) or will need surgery (when the patient is skeletally immature patients, large rib humps or lumbar scoliosis are factors that would suggest the convenience of the operation).