Subtracting minutes from a fixed time in a cell

[zekkk]

So, here's the problem.

I'll need to create a spdreasheet that must be able to do the following.
Insert the task that was done, the date, and how much time was spent to do that task.
The customer have 4 hours monthly that we give them, so we will do tasks for the customer based in this 4 hours. We go there, do the task, like 30, 25 minutes, and will insert on the spreadsheet, the spreadsheet must be able to when I insert this 20 minutes withdraw this minutes from the 4 hours. So far looks kinda simple and can be solved, BUT, the problem is that is monthly, so next month he will have 4 hours again, not cumulative. Next moth he/she will have just 4 hours, and we don't bill them for those hours.

Problem 2, in the same spreadsheet we must calculate this, when the 4 hours finish, we will calculate a 15-minute increment. Example:
4 hours have finished. Now we will do a 12 minutes task for the customer, the formula should count this: verify how many minutes and if is it smaller then 15 will consider as 15 and multiples per 0.75
If we do a task for them, and spend 16 minutes, the formula should calculate as 30 minutes and multiple that for the correct price, multiple for 0.75.

We give new customer a plus of 4 hours during the first month, so the first month the customer have 8 hours free.

I have attached the spreadsheet, so you can check what I've done so far.

Any small help will be good.