Anyway, after a recent incident of my frustrations with prolonged screen usage (the point at which I say "OK, you've been on long enough, you need to get off", and which they usually do respond to now), I realized that part of the reason they are on for extended times is they flit from one game to the next. I guess it's the equivalent of channel surfing on TV before the internet came along-you're not really actively watching anything, yet hours and hours get wasted sitting in front of the TV as you find one thing after another to watch, and sometimes you walk away thinking, hm, well, none of that was really worth watching. That gave me my idea for a new tactic-allowing screen time, but restricting it to one game at a time. When they are done with that one game, they need to do a non-screen activity first, before they can get back on (and not a 10 second non-screen activity...something of significant length of time). This seems to be a good balance, and sometimes, when they get caught up in that non-screen activity, they end up not going back to the screen entertainment at all! I haven't been restricting use by total time used or days on/off or anything lately, and this seems to be working in my quest to keep them from being on screens excessively!
Hopefully this will be a long term solution. It's not foolproof though, as I know they can easily spend HOURS straight playing Minecraft, but lately, they haven't been playing it much. I've dodged that bullet for now. But when they start up on Minecraft again, I think I might be OK with it as a once in a while treat. We'll see what happens if they start wanting to be on Minecraft every day though.