Robin, the Board of Regents of the University of Wisconsin have
commanded that we always request the maximum IDC the funding
organization will pay up to our HHS-negotiated rate.  Consequently we
always request the federal rate (for us at UW-River Falls, that is
currently 44% of salaries and fringe benefits) unless the funding
organization has a written policy saying they will only pay whatever, in
which case we request whatever.

There are obvious disadvantages to this policy, but it has the advantage
of being fixed.  I would worry that a sliding scale as you propose would
encourage grantors (and PI's) to always try to  negotiate a lower rate.  It
also would cast some doubt on the accuracy of your federally-negotated
rate.  If I were the feds, I would ask why you don't use a sliding scale
with my agency too?

Regards, Bill Campbell