Powershell Script to move 10GB of Exchange 2007 mailboxes

I want to be able to move a certain amount of mailboxes at night using a Powershell script.
The amount is up to but not exceeding my log file size which is currently 18GB.
I have about 40 SG/DB's and for various reasons they can grow.
I easily list mailbox sizes using something like:
Get-MailboxStatistics -database 'exmbox\sg4\db4' | Sort-Object TotalItemSize -Desc | Select -First 10 | ft DisplayName,@{label="TotalItemSize(KB)";expression={$_.TotalItemSize.Value.ToKB()}}, ItemCount
and can move some like this:
Get-MailboxStatistics -database 'exmbox\sg20\db20' | Sort-Object TotalItemSize -Desc | Select -First 2 | Move-Mailbox -TargetDatabase "exmbox\sg10\db10"

but what i don't know how to do is to count the size up to a certain size, putting each userid in an array to be perhaps piped into a move-mailbox command.

I also have some good example scripts for sanity checking, logging, e-mail reports but i lack the logic/syntax understand to do the counting part.  

Has anyone thought about this or have any suggestions??
Thanks, Michael

#############################
#                           #
#  Exchange 2007 Migration  #
#  Move-mailbox script      #
#  Date & Author            #
#			    #
#############################

# NOTE: to run PowerShell scripts, you need to set the PS execution poloicy "set-executionpolicy unrestricted"
 
# load the list of users - this must be 1 user alias per line
# the user list file must be in the same directory as this script
 
$users = get-content MoveMbx1.txt
# move em to a specific target database using 10 threads
$users | move-mailbox -TargetDatabase "MailboxServer\StorageGroup01\MailboxStore01" -MaxThreads 10 -confirm:$false
 
# put today's date in a variable
$date=get-date -uformat "%Y%m%d"
 
# read today's migration log and store it as $EMailBody
$EmailBody = get-content "C:\Program Files\Microsoft\Exchange Server\Logging\MigrationLogs\*$date*.xml"
 
# when done, send us an email with the log text
$SmtpClient = new-object system.net.mail.smtpClient 
$MailMessage = New-Object system.net.mail.mailmessage 
$SmtpClient.Host = "smtpserver.domain.com" 
$mailmessage.from = ("<emailaddress>@domain.com") 
$mailmessage.To.add("<emailaddress>@domain.com") 
$mailmessage.Subject = “MoveMbx1.ps1 script has completed” 
$mailmessage.Body = $EmailBody 
$smtpclient.Send($mailmessage)




# http://blogs.msdn.com/adamfazio/archive/2007/10/27/hey-exchange-2007-move-the-mailboxes-email-me-the-log-when-you-re-done.aspx

AND
Add-PSSnapin Microsoft.Exchange.Management.PowerShell.Admin 
  
$TargetDatabase = "Server\First Storage Group\Mailbox Database" 
$SourceFile = "c:\powershells\MoveMailbox\users.txt" 
$LogFile =  "c:\scripts\logs\MoveMailboxLog.txt" 
$error.Clear() 
  
write-output "*********************************************************************************************" | out-file -filepath $LogFile -append -noClobber 
write-output "" | out-file -filepath $LogFile -append -noClobber 
  
$UserList = Get-Content $SourceFile 
foreach($user in $UserList) 
{ 
     $message = "$(Get-Date) - Migrating User '$user' to " + $TargetDatabase 
     write-output $message | out-file -filePath $LogFile -append -noClobber  
     Get-Mailbox $user | Move-Mailbox -TargetDatabase $TargetDatabase -maxthreads 8 -BadItemLimit 5000 -SourceMailboxCleanupOptions DeleteSourceMailbox -IgnorePolicyMatch -confirm:$false  
     if($error.Count -ne 0) 
     { 
           write-output "$(Get-Date) - User '$user' failed migration" | out-file -filePath $logfile -append -noClobber 
           write-output "Error: " + $error[0].ToString() | out-file -filePath $logfile -append -noClobber 
           $error.Clear() 
     }  
     else 
     { 
           write-output "$(Get-Date) - Migration complete for user '$user'." | out-file -filepath $logfile -append -noclobber 
     } 
     write-output "" | out-file -filepath $LogFile -append -noClobber 
}

# XML file logging examples:
# Get-Mailbox ... | Move-Mailbox -TargetDataBase $DBtoUse -confirm:$false -maxthreads:10 -PreserveMailboxSizeLimit:$true -ReportFile "C:\temp\MoveMailbox\move.xml"
#
# Get-Content $SourceFile | Get-Mailbox | Move-Mailbox -TargetDatabase $TargetDatabase -maxthreads 8 -BadItemLimit 5000 -SourceMailboxCleanupOptions DeleteSourceMailbox -IgnorePolicyMatch -confirm:$false
#dir *.xml | foreach { 
#$xml = [xml](cat $_.fullname) 
#$xml."move-Mailbox".TaskFooter.Result | Select-Object @{n="MailboxName";e={$xml."move-Mailbox".TaskDetails.Item.MailboxName}},ErrorCount,CompletedCount,WarningCount 
#} 


# sample result 

#MailboxName ErrorCount CompletedCount WarningCount 
#----------- ---------- -------------- ------------ 
#joe bloggs1 0 1 0 
#joe bloggs2 0 1 0 
#joe bloggs3 0 1 0 
#
# http://powershellcommunity.org/Forums/tabid/54/aff/3/aft/4196/afv/topic/Default.aspx
#
# or filter
# ...|where {$_.ErrorCount -ne 0} 
# or
# ...|where {($_.ErrorCount -ne 0) -and ($_.WarningCount -ne 0)}

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