A family of Microsoft relational database management systems designed for ease of use.
How do the errant records differ from the others? If the adjusted 80% column can have a value of zero, then your expression would be attempting to divide by zero, which is a mathematical impossibility of course. If this is the situation try:
IIf(Nz([adjusted 80%],0) = 0,0,[New Plans]/[adjusted 80%])