Question regarding math behind fitness/fatigue

Can anybody help me explain the formula for calculating the daily fatigue and fitness?
I understand the concept of load, fatigue and fitness.

the question is more practical as it helps me better explain to colleagues and clients.

If i were to make an excel sheet, what would the formula for calculating fatigue and fitness be?

Here´s some ´readable´ interpretation

Search the forum for ´CTL Formula´ for more.

Try this thread: