Sharepoint list subtract available slots based on dates and time slots

[lras120265]

Hi, pretty new on PowerApps. But got a project, needs to keep track COVID-19 testing slots, 40 slots each day, 10 testing every two hours, from 8am -5pm. I was thinking to have two sharepoint lists. One collecting client info, the other list of dates, time slots and available slots. When one appointment is booked, it subtracts 1 from that time slot, I.e. from 10 to 9, till available slots becomes 0. What is the best approach ? Flow? Or use patch, updatecontent?

thanks!

[lras120265]

I know, I have two buttons on the same screen for "cancel/delete", and update. Customers/management haven't seen the application yet. It will be tomorrow or Tuesday. 

Thank you so much for inspiring me and helping me!