Rex, it's the controllers doing the asking. We aren't expecting you to do the impossible but are giving you the time and letting you work with your magic box to see what can achieved & going from there. Rather than some long spiel we just give the time and you come back with what you can achieve. I'll often add "let me know what you can achieve" if I think it's not possible on speed alone.
Personally if I think you can get it down to less than a pattern with speed I'll give you the time. I've seen some quite amazing time losses coming into ML from BN (into the wind I guess) when I was expecting a 4 or 5 min vector.
ACMS, an accurate estimate is great, but it won't make an iota of difference because it takes no account of all the aircraft who have yet to depart who will be getting there before you.
400NM out? The controller who will be fiddling with Maestro may very well not even be aware of you at that stage and is busy working the sequence and handling their other duties for the aircraft they already have. As a pilot are you thinking about your next leg rather than briefing for the arrival you're about to make? That's what you're asking for. It's not the way Maestro works.