There really isn't anything to calculate unless your tripping your main breaker with what you have. Panels are ctl meaning they are circuit limited so if the panel is full you may have a few options. We can tell you but need the following questions answered.
Can you tell us what is the manufacturer of the 150 amp panel you have?
Can you also give us the model number? ....this will usually be on the specifications sheet on the inside of the panel door.