calculating micro controller power consumption
Just go to www.atmel.com and type in ATtiny45-20PU in their search bar. There you will find the ATtiny "Complete" datasheet instead of the "summary" you found at Mouser. 

The complete datasheet of an MCU or CPU is of course not a paper you read from start to beginning. 

Hint: 
Go to the DC Characteristics at page 161, 162 and see if you can sort out the current consumption from there.