New to PS - Need to use calculation as object

I am new to powershell and have been struggling with a script for a week or two now.

I am trying to get powershell to obtain disk information on a machine, then analyse the percentage free and write and event to the eventlog if a certain condition is met.

So e.g

check the disks and if the capacity <400GB and percent free is <10% then write an event(i have the event log write bit working) to the event log.  Our monitoring system picks up event logs and can perform actions based on them.

However, i also want the following:

If capacity is >400GB and percent free is <10 and free space is less than 40GB then write an event to the event log.

I am able to calculate the percentfree using the following:

{Name="percentFree";Expression={[int]($_.Freespace/$_.Size * 100)}}

however, i dont seem to be able to create a $percentfree object from the abpve calculation that i can then use in the IF statement.  

Using the above, to test, i used 

Write $percentfree 

it simply writes the above expression.

Any help would be greatly appreciated.  please be Gentle, i am new to this.

Thanks