At 40 Centigrade ambient Temperature at which percent Loading power transformer should be operated maximally?

High ambient temperature may result in High Oil Temperature as well as winding temperature of the transformer if the load is considerably high. But I wouldn't consider surrounding temperature as factor for loading power transformer.

The transformer can be operated in 85% loading or above if the winding temperature is not that high  in order to meet the load requirement. It's upto the operator to decide if the load is to be reduced, and offcourse the load should be reduced as the winding temperature rises, it should be maintained below 60 degree centigrade, the transformer fan gets ON soon as the temperature rises above 60 degree (the temperature can be calibrated to any value/ but usually set to 60 degree for fan ON, 95 degree for Alarm and 105 degree for tripping) and if the temperature is rising further the load should be reduced so that the temperature will also reduce to a lower value.

To be precise and brief 85%

The transformer cab be operated at 85% of loading in peak summer season

The Transformer should not be operated above than 80 % in Hot summer weather. The winding temperature will increase which will resultantley dectrease the life of transformer insulation.