No fixed formula can be laid for fixing the amount of maintenance. It has to be in the nature of things which depend on various facts and circumstances of each case. The court has to consider the status of the parties, their respective needs, the capacity of the husband to pay, having regard to reasonable expenses for his own maintenance and others whom he is obliged to maintain under the law and statute.
The courts also have to take note of the fact that the amount of maintenance fixed for the wife should be such as she can live in reasonable comfort considering her status and mode of life she was used to live when she lived with her husband. At the same time, the amount so fixed cannot be excessive or affect the living condition of the other party. These are all the broad principles courts have to be kept in mind while determining maintenance or permanent alimony.
-excerpts from Vinny Parmar vs. Paramvir Parmar
I hope this helps.