Recently found an interesting article how to calculate Raahukaalam. It depends on the Sunrise and sunset in that area on that day. (Sunset-sunrise) is the day and it is divided into 8 parts. Each part is called period and Raahukaalam comes in one of the periods in a day. There is no Rahu Kalam during the first period of any weekday. On Monday it falls on the 2nd period, Saturday 3rd period, Friday 4th period, Wednesday 5th period, Thursday 6th period, Tuesday 7th period & Sunday 8th period.
But, I don't have idea why they say so.