Rahukaalam and Yamakandakakaalam etc. as given in most panchangas and calendars are only general and presumes that sunrise is at 6.00 am and sunset is at 6.00 pm. They vary from place to place and time to time.
To find the actual raahukaalam, first find out the day duration for the particular day ie:- number of minutes from sunrise to sunset. ( eg:- sunrise 6:30 am, sunset 5:30 pm, day duration would be 660 minutes). Divide this into 8 portions of equal duration. In the above case, each portion will be of 82.5 minutes. Take the 8th portion on Sunday, 2nd portion on Monday, 7th portion on Tuesday, 5th portion on Wednesday, 6th portion on Thursday, 4th portion on Friday and 3rd portion on Saturday. In the above case, if it is a Sunday, raahukaalam would be from 04:07:30 pm to 5:30 pm (ie:- 8th portion), if it is a Monday from 07:52:30 am to 09:15 am and so on.
In the case of yamakantaka, the same calculation is to be performed first. Take the 5th portion in case of Sunday, 4th portion for Monday, 3rd portion for Tuesday, 2nd portion for Wednesday, 1st portion for Thursday, 7th portion for Friday and 6th portion for Saturday.
In the case of Gulika kaala, perform the same calculation as above. Take the 7th portion on Sunday, 6th portion on Monday, 5th portion on Tuesday, 4th portion on Wednesday, 3rd portion on Thursday, 2nd portion on Friday and 1st portion on Saturday.
