Thanks for the suggestion!
I’d like to be able to use the per-goal “days ahead” number to help me prioritize. (My main Beeminder interface is a modified version of @alys’s command-line UI, and I’m thinking of adding an option to sort by “days ahead” if I can get the metric to feel right.)
The place where I feel this most is with working hours (which I track with weekends-off enabled). I like to stay about 3 days ahead, ideally. If it’s Wednesday, this will show up as having a 5-day safety buffer, so unless I consciously remind myself that this includes the weekend, I feel like I’m further ahead than I actually am and risk dipping into my buffer. If I’m measuring days ahead, I can try to keep this at 3 all the time.
Taking a far-future rate into account messes this up: I have to consciously remind myself that it’s using a different rate, so it’s less useful for prioritizing what to do today.
I think the rolling-window strategy will fix this for my purposes (but I’ll have to see how it looks when I get it going). My hope is that by setting D somewhere around 10-20ish, it’ll be long enough to capture whatever “real rate” you’re using at the time but short enough that the individual goal values feel relevant. (Something to watch out for is a break of more than D days, which could break it.)