An overflow occurs when you assign a data type as one type and that data type cannot store that data because it is too large for the 'container' you assign. For instance you can do:
Dim byt as Byte
byt = 255
but you can't do:
byt = 256
or
byt = 25.5
because a byte data type will only hold integer values up to 255, to correct the above errors change the variable declarations to the appropriate types. Do a search on Data Type Summary in Access Help for direction as to which data type is appropriate.
In your case it sounds like your formula is returning a fractional value and you have the fields data type set to Integer or Long, try changing it to Single or Double and see how you get on.
Ian