Profitable Trade - Microsoft (MSFT) Puts

Today our Microsoft puts expired worthless. Our transaction was as follows:

STO 4 MSFT Dec 17 2011 24 Puts@.38               +138.95

Our return is 1.45% for 25 days which equates to 21.17% annual. If one were to calculate return on margin (ROM) our return on a margin requirement of $1744.91 is 8% which equates to 115% annual.