How to find maximum coil current for relay based on nominal current value given in datasheet
You should not do this. Assuming 150°C insulation the temperature rise at 85°C ambient is 65°C with 16V applied. Refer to figure 3 on the datasheet. 

Even if your ambient temperature never exceeds 25°C (so the allowable temperature rise would be 60°C more), the maximum allowable voltage would be 

Vmax = 16V\$\cdot \sqrt{\frac{125°C}{65°C}}\$ = 22VDC (Ta \$\le\$25°C). 

The assumption in the above is that temperature rise is proportional to coil dissipation. 

Use the proper voltage relay or add a ~1W resistor in series with the coil. 



Note that the resistance of the coil varies significantly from room temperature to the maximum coil temperature, however at the maximum it is roughly the same, whether you get there from high ambient temperature or from heating due to over-voltage.