Available Kms per week

Hi all. I am a coordinator who works with people with disability. One of the sticky problems is tracking available Kms for these participants each week. Commonly these participants are allocated a budget for travel for the extent of their service agreement and that is divided by the amount of weeks available in that agreement. But travel is a very inconsistent variable. I have attached a spread sheet that has sample data with the piece I can't work out highlighted. We have support workers daily calling in to ask if travel is available this week and it has been a constant struggle to give an accurate answer. Any help is appreciated. The attachment has some simple formulas within it. All data is fictional.