The way that the VA combines percentage ratings is complicated. I understand why clients get confused when the VA does something like adding two 30% ratings to get a total rating of 50%. It won't help you understand things, but the VA does have a chart that shows the basics. You can find it here.
Essentially, the VA subtracts the biggest rating and then takes each additional percentage off of what's left. So, when "combining" two 30% ratings, the VA first subtracts 30% from 100% and decides the veteran still had 70% of the ability to work remaining. The second disability only takes 30% off of that 70%, which means another 21%. Thirty plus twenty-one makes the Veteran 51% disabled. The VA rounds to the nearest tenth and puts that at 50%.
These are just the basics and there are a lot more rules, however. At some point, I'll probably write about the rule that means 0%+0% can sometimes equal 10. If you are confused by your combined ratings, the best advice is to consult with someone experienced in handling veterans benefits.