Cholesterol comes in many forms in the body and it is almost never measured directly. As mentioned above 20% of chylomicrons carry cholesterol and since most of triglycerides are in chylomicrons the approximation is made.
The levels at which to treat depend on the number of risk factors for coronary disease. It was though that the high HDL would decrease the level at which one treats high LDL, but this view in now less popular since there are disorders which cause high HDL and also high rates of coronary disease.
So ask you doctor what your risk factors are and whether or not you need treatment. To be safe you could start a cholesterol lowering medication with any number of risk factors, but even then the goal for treatment is different.