Prosca - take a half tablet 3 times a week??

Hi, I switched to Prosca from Propecia. My doctor instructed to split Prosca into two and take each piece Monday, Wednesday, and Friday. I know many people one tablet split into 4 pieces and take a each piece everyday but never heard split into two and take one piece 3 times/ week. If I do the dosage per week will be 7.5 mg and because I lose some when I split it makes sense but I just never heard anybody doing this. Also I heard if I split it and leave in air the effectiveness will reduce. Any experience and comments?:cool: