Round Number to Two Decimal Places 
[math]::Round($number,2)
Details
Round number to two decimal places. Change the number after the comma to round to different decimal place

Example
PS C:\> $number = 3.14285714285714
>> [math]::Round($number,2)

3.14
  |  
Round to the Nearest Half 
[math]::Round($number * 2, [MidpointRounding]::AwayFromZero) / 2
Details
Round a number to the nearest half (.5).

Example
PS C:\> $number = 3.64285714285714
>> [math]::Round($number * 2, [MidpointRounding]::AwayFromZero) / 2

3.5