How do I know I'm not overloading my sub Panel? What is the calculation for determining my load and what size main over current device should be used?
Your sub-panel came with a maximum amperage rating. The breaker that feeds the panel must have no higher rating than either that or the supply cable.
Is the breaker feeding the sub panel the correct size for the panel and cable ratings? Is that breaker tripping?
If the answers are yes and no then you have no problem.
Keep in mind that the sum of the breakers installed has nothing to do with overloading of any panel.
It is very rare outside of an industrial setting to have even a fraction of the circuits sullying their full load.