I think this is highly dangerous and for me there no non-zero % that wouldn’t have me thinking “If I take this derail before end of month I pay less on my next premium bill.” It encourages you to derail sooner to keep your bank fat, because any premium bill that gets paid with non-bank funds is a permanent loss.
I maintain that my original suggestion (Why should you pay for beeminder premium? - #4 by drtall) of having premium count towards pledges is much less destructive. It encourages you to derail later, when your bank is fatter and the total bank size is finite in a given time period.