Adjusting work based revenue at end of fixed price contract

We are starting to use work-based-revenue.  We have a billing extension that uses a common set of burden schedules and a top task level 'multiplier' to determine the amount of revenue to accure.  Many projects share the same burden schedule, but due to differences in cost budgets and sales price, we found we need a 'multiplier' on top of that.

Without going into a lot of detail, that is generally working well.

However, a majority of our projects are fixed price, and of course the amount of revenue accrued at the end of the project is never perfect.    We can use the hard limit to keep the revenue from going over, but what do we do when we have a bit of revenue left to accure?  Any suggestions?  It doesn't seem reasonable to expect the users to adjust burden schedules or rates at the end of a project to make the numers come out perfectly.  We have many small projects, as opposed to a few large ones, so that would require way