If you are wanting to know what time it is in India when it is x time at Greenwich, there are no calculations really required beyond the hour adjustment between the time zones. But if you are asking about the longitudinal difference for an astrological calculation, the longitude is required for the place being computed. It's generally a minute for each three degrees and 56 minutes of longitude, multiplied by the difference between the India longitude and the Greenwich longitude. This is the purist form of calculation which does away with having to look at a table. It does require that you take the distance and convert it into minutes, then take the longitudes and convert them into minutes, so that all the calculations are performed with the unit of minutes of longitude. Then once you have the distance measured you would convert it to the degrees and minutes for the table lookups.

 IST is 5.5 hours ahead of Greenwich Meridian Time. It is 3:54 pm in India and 10:24am according to Greenwich Meridian Time.