Let's say underlying is $50 and a put with ten days to expiration is selling for $1. Do you annualize based on calendar or trading days (2%*~36 or 2%*25)? Now assume you're supposed to use calendar days and wanted to use a formula that takes compounding into effect. Is this the proper way to do it...(1+(1/50))^36-1) = 1.04 i.e. 104%?