Attendance Management - employees are punching late - notice chart on how the In -out will be calculated in pro-rata basis

Hi to all.. I have a small query that we have working hours of 8 hrs including lunch break. Some of the employees are punching late from half day, so working time for those late punchers is only 3 to 3.5 hrs. I want to clarify that all the employees that how the In -out will be calculated in pro-rata basis. So just give me a brief and clarify my doubt and it will be good if i get notice and chart to present it in notice board.
Thank You.

Lost of man hours in the referred cases almost 50%. Does this happen as routine or rarely? Since you have punch card system it's obvious only for the half shift they really worked. You have to formulate your HR policy reg.dealing with late attendance/leaving early, get them approved/validated by competent authorities. If you have a valid SO check whether there are enough provisions therein to deal such irregular punching. Based on this status you may take action deemed necessary.