this reminds me very strongly of a custom goal/reminder setup i helped a lovely user with awhile back (july 2015, where does the time go?!).
the ultimate goal was “keep a hard limit on my inbox age - no items older than X days in my inbox at any time.” like you, he had an autodata system set up to send that oldest value every day. it was essentially a flat whittle-down (do-less without autosum).
the key insight to coming up with a useful reminder system was that the hard cap and not the countdown is closer to the actual “time until derail” value you want to be paying attention to (if you’re tracking in days, which you probably should be b/c beeminder operates in days). so, for example, if your goal limit is “no items older than 7 days” and your oldest item today is 5 days, your hard cap = 2. in 2 days, your oldest item will be 7 days, and you’ll have to take action the next day to avoid derailing.
(then he set up a google apps script to label and color the beeminder reminder emails accordingly, based on the hard cap value in the subject line. this was quite a system. BUT the goal is still in use today, so it’s doing its job!)
so, ideas on the beeminder feature side of things: allow you to enter a daily default increase rate (vs. assuming one standard for all goals), then calculating the countdown off that could be a start!
radical alternative: ditch the countdown entirely for do-less goals, since it often doesn’t really make sense and causes confusion. the only rule is no more than the hard cap. you just get reminders every day telling you to stay under your hard cap.