If 10 mg of prednisone for x amount of days helps me during a debilitating lupus flare, does that mean my case is mild or are some people just really sensitive to prednisone?

Everyone differs. Everybody's lupus is different. You may have some symptoms but you may also may have some manifestations you don't even feel. Your physician can better able answer your question. This depends on what symptoms you have that are active lupus, plus what manifestations you have you can't feel.
No correlation. Some people get better systemic responses than others on a relatively low dose of prednisone. I am banking on the likelyhood that your flares have more to do with pain and fatigue: these are very steroid resonsive symptoms. 10mg of prednisione will have a neglible effect on vasculitis associated with sle.