The pledge schedule has probably been discussed to death by now, but whatever. Beeminder has a logical explanation for why they use an exponential pledge schedule. But I am irrational. My dumb brain doesn’t really care all that much when I derail on a $30 pledge, even when my bank account is nearly empty. On the other hand, paying $90 feels unthinkable. Similar to how the brain perceives $9.99 to be significantly less than $10.00, I think I subconsciously round $90 up to $100 and $30 down to zero. This means that derailing on a $90 pledge once feels way worse to me than derailing on a $30 pledge three times.
For me, something like $40 would probably be the sweet spot between “low enough to consider pledging” and “high enough to deter heavy akrasia”. (Though I suspect I’m significantly less frugal than most users.)