From my experience of having consulted on seven competitive bids, I cannot stress too much the crucial step of identifying assumptions. The proposing law firms and the law department should both state as many assumptions as they can and how they are relying on them. For example, “our bid assumes no more than four investigations in any 12 month period.” Only by doing that can both sides narrow the range of uncertainty. Thus, if the law department says, “No, assume no more than two investigations,” the law firms can more accurately bid on the work.
It is best to circulate all assumptions to all proposers, so that the information level remains equally high all around.