So it does sound like before school is not going to be an option. Maybe on school days during the winter 5pm - 7pm was o.k., however, I would push it closer to bed time on non-school days and the Summer. Those can be some prime family time or helping with supper or outdoor time for non-school days. That is where you will run into the problems of "But you said" and you are trying to say, "But we want to do ...... tonight". Make easy child time as far away from any other possible activities as you can. Usually later in the evening when you KNOW that no one in the family will be running anywhere and you will be less likely to ask anything of difficult child. I would have it end atleast 1/2 hour before bedtime. (I hope this is making sense)
I do like how you deduct time used over and beyond from the next time slot.
If he has done well during the week, I would consider allowing him to have more time on Saturday mornings with the rule that once YOU are up for the day the easy child gets turned off and he joins the family in Saturday morning responsibilities, errands, exercising, whatever you have planned for that day. It will be hard for you to regulate otherwise. As long as he is not disturbing the household, let Saturday early morning be his free time. Maybe don't even make him pay for the easy child or other quiet activities he chooses to do before the family's day begins on Saturdays. We all have to feel some freedom in our lives.
I am leaving tomorrow morning for our Easter family trip. I don't know if I will have a chance before Sunday evening to check in again. Have a GREAT weekend.