Help with excel formula for percentage

Hello, I am trying to write a formula to calculate the percentage difference between planned time vs actual time. For example I have a planned time of 10hrs and 30 minutes but an actual of 8hrs and 10 mins. Now I know that there is 2hrs 22 mins difference and I have shown that in time format in excel. What I need to do is show that time difference as a percentage. The reason for having the percentage calculation is because if any actual times are not over 75% of the planned times then there is a penalty that gets paid back to the contractor for failure to complete hours. Can anyone provide any help or advice with this. Many thanks