Maybe I'm getting this wrong but... you're working 3/5 of the time, you're getting 1/2 of the money and you are earning the company > 100% than full time?
I'm working 1/2 the time (7 hours Monday & Tuesday, and 6 hours Wednesday), and, yes, it even surprised me that my total productivity is about the same as when I worked 40 hours per week. It's even possible that my total productivity is actually greater than before but we don't have sufficiently precise metrics off of which to judge that.