I have a script that clones a bunch of VM's from a text file from 1 location to a specific data store and it works great. I need to incorporate a validation check at the start to confirm the combined VM's from the text file will fit on the datastore OK. I'm totally stuck at the highlighted point below in bold can anyone kindly give me a clue 
Would be extremely grateful if someone could give me a pointer
The below is what what I have so far...
# ==============================================================================================
# Varibles
# ==============================================================================================
# Add the vmware snapin for powershell
Add-PSSnapin VMware.VimAutomation.Core
# backup = true appends date; false creates a clone with the same name.
$backup = "True"
# debug - true : will not clone vm; FALSE will clone the vm.
$debug = "FALSE"
# Target Datastore
$targetdatastore = "Synology LUN1"
#Refers to the percentage of free space for the DS space check
$freePerc = 5
# Target location - existing folder in vcenter structure, where the clones will be stored
$targetlocation = "TestTarget"
# Set date
$datestart = (get-date -uformat %Y-%m-%d)
# Name a logfile to capture results.
$logfile = $datestart + "_VMClones_bulk.txt"
write-output "New Log ($datestart) - ($logfile)" >> $logfile
# Establish Connection
Connect-VIServer -Server 127.0.0.1 -User administrator@topsecret-Password topsecret
# ==============================================================================================
# Validation checks Prior to starting the clone script
# ==============================================================================================
$ds = Get-Datastore -Name $targetdatastore
# Validation check 1, Does the combined provisioned total in GB from $VmListFile exceed the capacity of the destination datastore?
if(($ds.FreeSpaceGB -lt $VmListFile. Combined size of $VmListFile )
{
Write-Host "Continue with your script"
}
else
{
Write-Host "There is insufficient space on the target datastore. Please increase Space to continue"
Write-Output "There is insufficient space on the target datastore. Please increase Space to continue" >> $logfile
Write-Host "Exiting with code 1"
Write-Output "Exiting with code 1" >> $logfile
exit 1
}
$ds = Get-Datastore -Name $targetdatastore
# Validation check 2, do you have X % free datatstorespace?
if(($ds.FreeSpaceGB/$ds.CapacityGB*100) -ge $freePerc)
{
Write-Host "Continue with your script"
}
else
{
Write-Host "There is less than X% space free on the datastore. Please increase Space to continue"
Write-Output "There is less than X% space free on the datastore. Please increase Space to continue" >> $logfile
Write-Host "Exiting with code 2"
Write-Output "Exiting with code 2" >> $logfile
exit 2
}
$VmListFile = Get-Content VmsToCloneList.list | Select-Object -Skip 3
ForEach ($vmname in $VmListFile)
{
# The VM Object
$vm = Get-VM -Name $vmname
# Target Host - use the same host as the current VM ( this is faster than cloning across hosts ).
$targethost = $vm.vmhost.name
# Target VM Name - name if BACKUP is FALSE
$vmtarget = $vmname
#
$datastore = get-datastore $targetdatastore -vmhost $targethost
# ==============================================================================================
# Begin the actual cloning script
# ==============================================================================================
if ($vm -ne $null)
{
Write-Host "A VM named $VM exists. Starting clone"
Write-output "A VM named $VM exists. Starting clone" >> $logfile
}
else
{
Write-Host "A VM by the name of $VM perhaps it does not exists"
Write-Output "A VM by the name of $VM perhaps it does not exists" >> $logfile
Write-Host "Exiting with code 2"
Write-Output "Exiting with code 2" >> $logfile
exit 2
}
if ($backup -eq "TRUE")
{
# Clone the VM to backup_vmname_todaysdate
$vmtarget = "backup_" + $vmtarget + "_" + $datestart
}
# nice colors if you are watching the script run
write-host -foregroundcolor green "Cloning $vm to $vmtarget"
write-output -foregroundcolor green "Cloning $vm to $vmtarget" >> $logfile
new-vm -name $vmtarget -vm $vm -vmhost $targethost -datastore $datastore -Location $targetlocation -DiskStorageFormat thin
}
# COMPLETED
Its a combined size of several VM's mate which complicates things a tad. I came up with this but unsurprisingly it doesn't work
$VmListFile = Get-Content VmsToCloneList.list
$ds = Get-Datastore -Name $targetdatastore
# Validation check 1, Does the combined provisioned usage of the VM's exceed the target destination datastore?
if(($ds.FreeSpaceGB -lt $VmListFile.UsedSpaceGB))
{
Write-Host "Continue with your script"
}
