round up to the next 1/2 kg....formatting

[fishqqq]

I have a field that updates the chargeable weight of shipments and
this field returns a number with 5 decimal places

The formula works perfectly except i need to round the weight up to
the next 1/2kg

ie:
325.24591 should be rounded to read 325.5
325.61111 should be rounded to read 326.0

currently the control source of the field is set as follows:

=IIf([text149]>[text151],[text149],[text151])

and what it is doing is comparing the actual weight of something to
it's volume weight and displaying the higher of the two. What i need
now is for the higher of the two to be rounded to the next 1/2kg as
shown above.

Can anyone recommend how to do this?

thanks
Steve