Maybe cause it's Friday but I am totally stumped here!

In my new role, staff will add x amt of hours to their shift as overtime. For example, John is scheduled to work until 11:00am. He emails to say "I will pick up 3 hrs 15 min of overtime at the end of my shift" can I not put in a formula that says if you add 3.25 hrs to an end time of 11am, he will be working till this time. Also in the reverse where if he starts at 8am and wants to work 3 hrs and 15 minutes overtime before his shift, then a formula where his start is 8am and 3.25 hrs before is a new start time of....

I think the snow in our town is freezing my brain!