How do I know how many significant digits I need in my uncertainty?

I am just about done the data processing portion of my lab, and I'm not sure how many sig digs I need to put in my final answer's uncertainty. Right now, I have 55.18% uncertainty with my final answer of -5.64MJ. This lab was dealing with Enthalpy of the combustion of wax. Also, should I be calculating percent error or percent difference for my lab? We are given the theoretical value and then we have our own value so which one do I use??