I wish to create a cell that gives a rounded up/rounded down figure i.e.

Assume figure of 20.38

The number before the decimal point will be variable.

The number after the decimal point will be subject to the following
conditions:-

If < .124 then round down to next whole 0.5 (if 20.38 then figure to be 20.0)
If .124 then round up to next whole 0.5 (if 20.38 then figure to be 20.5)
If < .624 then round down to next whole 0.5 (if 20.45 then figure to be 20.5)
If .624 then round up to next whole 0.5 (if 20.72 then figure to be 21.0)

How do I write the formula please?