I think it has already been handled, but let me try to clarify.
If the person doing the cashup, has the permission to "Change Float", the last screen of the cashup will allow you to enter the monies you leave in the till - the new float. This is for many of our customers that do not wish to take the change out of the till / drawer & they will simply enter the notes that they leave behind. This will make up the new float.
The easy answer is then to ensure that all cashiers do not have permission to change float & it will then stay constant & easier...