It depends on the discretion of judge as to how he arrives at the figure, taking various factors into consideration. However your question is very intelligent and appreciable. Income alone cannot be criterion for deciding the maintenance amount. Take home salary and also the expenses also should be taken into consideration while awarding maintenance to wife. It is another thing is she is staying at home because living at same house means less expenses on food, rent etc. But if she wants to live outside, it will have a bearing on the expenses of the husband because the expenses increase. So in a case where husband is getting a salary of Rs.15000/- and spending Rs.14500/- per month which includes a rented house of say Rs.4000/-, if the court gives maintenance following Supreme Court order saying wife is eligible to live same standard of living that she used to live while she was with her husband, then the rent alone would amount to Rs.4000/- to wife. And supposing other expenses on food, clothing are assessed at Rs.2500/- than the court awards Rs.6500/- to wife as maintenance to ensure that wife lives in the same standards that a husband lives. Which means husband is left with hardly Rs.8500/- and he will have to kill his mother without giving her medicines.
Hence, to say that I want separate accommodation and same standard of living and courts supporting that line is cruelty.